In a large stock pot, melt 2 tablespoons of butter. Add chicken and cook over medium heat stirring often, until chicken is no longer pink. Remove chicken onto a plate and set aside.: Close your eyes and listen to the light sizzle as the butter melts and the surface of the chicken begins to turn opaque, that faint hissing is the Maillard reaction starting to develop a savory base. The smell of warm butter and browning meat signals flavor being locked in. Move the pieces frequently so they brown evenly rather than sticking and burning, because uneven heat will create bitter spots. I always watch for the edges to lose their raw sheen but remain juicy, then transfer the chicken off the heat to avoid overcooking which dries the meat. A common mistake is leaving the chicken in the pot until it is completely firm, which makes it tough after the simmer, so pull it early and let it finish later in the broth.
To the pot, add onion, garlic and mushrooms and cook, stirring often, until onion is almost translucent.: As the onion hits the warm fat, you will hear a gentle sizzle and smell a sweet, toasty aroma as sugars begin to melt out. The mushrooms will release their earthy juices and shrink, while the minced garlic becomes fragrant quickly, so stir constantly to prevent scorching. This step creates the aromatic backbone that flavors everything else, so take your time until the onion softens and becomes slightly glossy. A typical error is turning the heat too high which browns the aromatics too fast, giving a bitter char instead of mellow sweetness, so keep the heat moderate and patient.
Add potatoes, celery and carrots and stir all vegetables together.: When you toss in the diced potatoes , celery, and carrots , you will notice the pot filling with a heartier texture and a subtle vegetal scent. Stirring coats the pieces in butter and aromatics, starting the gentle toasting that adds depth once the liquid is introduced. Listen for the soft thud of the veggies as they move in the pot and watch for a slight sheen as they pick up the fat. Avoid overcrowding, if the pieces are clumped they will steam unevenly, so spread them uniformly for even cooking.
In a mixing bowl, whisk together milk, chicken broth, flour (or cornstarch) and Italian seasoning. Add to pot with vegetables. Stir together. Bring to boil. Reduce the heat and simmer.: Whisk the milk , chicken broth , and thickener until smooth so the liquid becomes a unified, pale slurry; this prevents lumps and creates a glossy finish. Pour it into the pot and you will immediately see the mixture envelop the vegetables, turning into a silky broth. Bring it up to a rolling bubble to activate the thickening power, then lower the heat so it simmers gently, maintaining delicate steam and small ripples. The gentle simmer lets the starches soften and meld flavors; a common oversight is to boil too hard which can break down textures and make the milk taste cooked or curdled, so keep it calm and steady.
Cook until potatoes and carrots are soft, about 12 to 15 minutes.: During this simmer you will see the broth thicken and cling to a spoon, and the aroma will deepen into a warm, savory scent that fills the kitchen. Check the potatoes by piercing with a fork; they should slide off easily but not be mush. The carrots should be tender and sweet, adding gentle texture. Stir occasionally to prevent sticking on the bottom, and adjust heat if the liquid is bubbling too vigorously. Overcooking here causes the vegetables to fall apart and thicken the broth excessively, so test early and often for the right bite.
Add peas and cooked chicken and stir together.: When you fold in the frozen peas and the reserved chicken , the pot will come alive with pops of color and a renewed savory perfume. The peas will warm quickly, keeping their bright green hue and slight snap, while the chicken absorbs the seasoned broth and becomes juicy again. Stir gently so you do not break the pieces apart, and simmer just long enough to heat through. A frequent misstep is leaving these in too long which makes the peas dull and the chicken dry, so aim for a brief finish.
Serve with fluffy drop biscuits.: Ladle the soup into bowls and pair with warm, fluffy biscuits for a classic combination where the bread soaks up the creamy broth. The contrast of soft biscuit and velvety soup creates an indulgent mouthfeel that is deeply satisfying. Present it steaming so guests can enjoy the aroma, and encourage dunking for maximum comfort. Avoid serving cold biscuits, which will cool the soup and lose the comforting contrast that makes this pairing feel special.