Set a large skillet over medium heat. Add the butter. Once melted, whisk in the flour until smooth. Whisk and cook the roux for 2-3 minutes, until golden.: The moment the butter melts you'll smell a buttery sweetness, and as you add the all purpose flour the mixture will transform into a satin paste that begins to smell toasty when cooked. Stir continuously with a whisk so the roux heats evenly and doesn’t form lumps, moving the spatula in smooth circles until the color shifts to a light golden hue. This toasty aroma signals that the raw flour taste has cooked away, which is essential for a glossy, flavorful gravy. A common mistake is rushing the heat to hurry the process, which can brown the roux too quickly and introduce bitterness; keep the flame steady at medium so you develop flavor without burning. The texture should be smooth and slightly thick on the whisk before you add any liquid, indicating the flour has been properly incorporated.
Then stir in the onions. Cook the onions 2-3 minutes, until soft. Then whisk in the water, chicken base, and poultry seasoning. Simmer 2-3 minutes to form a gravy.: As you fold the chopped onion into the roux, the kitchen will fill with a sweet onion scent as the pieces become translucent and tender. The onions will release moisture that helps loosen the roux into a sauce base, so watch for that glossy sheen. When you whisk in the measured water , drop in the chicken base or bouillon and the poultry seasoning , the mixture will thin and begin to steam, releasing a savory, herbaceous steam that tells you seasoning is integrating. Let it simmer gently for a few minutes until the gravy is smooth and slightly thickened, coating the back of a spoon. This step matters because the gravy is the binding element that clings to the chicken and vegetables later; if you undercook it the sauce may be thin. Avoid boiling aggressively which can break the sauce and dull the flavors.
Set out a large 6+ quart slow cooker. Pour the gravy into the slow cooker. Add the diced chicken, mixed vegetables, and parsley. Stir well.: When you transfer the warm gravy to the slow cooker you'll notice a comforting steam and the glossy sauce settling into the bowl. Add the diced chicken pieces so they are submerged in the gravy for even cooking, then scatter the frozen mixed vegetables across the surface; they will release tiny beads of moisture as they begin to thaw, which helps the overall texture. Sprinkle in the chopped parsley for fresh herbal lift. Give everything a gentle stir so the gravy coats each piece evenly; this ensures uniform seasoning and prevents pockets of dryness. A typical oversight is overcrowding with too much chicken without enough sauce, which can lead to dry meat, so keep the proportions balanced and stir to combine.
Cover the crock and set on HIGH for 3-4 hours or on LOW for 6-7 hours.: As the slow cooker hums you will hear a faint, steady simmer and smell the filling deepen into a rich, savory aroma. On HIGH the dish will reach tender doneness faster, with the chicken becoming pull apart tender in about three to four hours; on LOW the flavors develop more gently and the texture becomes silkier after six to seven hours. The long gentle heat helps the gravy thicken slightly and the vegetables soften without turning mushy. Avoid lifting the lid frequently, because every peek lets out steam and lengthens cooking time, which can affect timing and moisture. Trust the slow cooker to do its work and only check near the minimum recommended time to test for doneness.
Close to dinner time, preheat the oven to 425 degrees (or the temperature listed on the can of biscuits.) Use kitchen shears to snip 5 biscuits into 8 pieces. Spread the pieces out on a baking sheet. Bake for 11-12 minutes, or until golden.: The hot oven will inflate the canned biscuits into puffy pillows of golden crust, giving off a warm, bready fragrance that signals they are nearly ready. Snipping each biscuit with kitchen shears into uniform pieces encourages even browning and makes them easy to scatter over the filling. Arrange the pieces with space between them so air circulates and the edges crisp up; crowded pieces may steam and stay pale. Watch the oven window for a deep golden color and a hollow sound when tapped, which indicate doneness. A common slip is underbaking to save time, which yields dense interiors, so allow the full bake time for a light, airy result.
Once the biscuit pieces are puffy and golden, stir the chicken pot filling and sprinkle the biscuit topping over the filling. Serve warm.: When you lift the lid and stir the pot the filling should be glossy and cling lightly to each piece of chicken and vegetable, releasing a rich steam that smells homey and satisfying. The biscuit pieces should be golden and slightly crisp at the edges; scattering them over the filling creates a delightful textural contrast as some pieces sink and soften while others remain crisp. Serve immediately so the biscuits retain some structure while the filling stays warm and silky. One mistake to avoid is assembling too far in advance, because the biscuits will become soggy over time; serve right after topping for the best experience.