Cook the pasta according to package instructions.: The pasta should be cooked until it is al dente, offering a firm bite that holds up when tossed with the sauce. You will notice the water smell becoming starchier as the pasta releases surface starch, which actually helps the sauce adhere. Reserve a cup of pasta water if possible, it can rescue a sauce that is too thick, and avoid rinsing the penne pasta because rinsing strips the starch that helps the sauce cling. A common mistake is overcooking, which leads to mushy texture and a flat mouthfeel.
Meanwhile season the diced chicken thighs with salt, pepper, paprika and garlic powder. Heat 1 tbsp olive oil in a large skillet or dutch oven over medium high heat. Cook chicken in skillet, until browned and cooked through, transfer chicken to a plate to keep warm.: As the chicken hits the hot oil you will hear a satisfying sizzle, and the surface will develop golden brown crusts that concentrate flavor. Browning creates the Maillard reaction, which adds savory depth to the final dish, so resist the urge to move the pieces too frequently; let them form color. Transfering the chicken off the heat prevents overcooking while you prepare other components, keeping the meat tender. Watch out for overcrowding, which causes steaming instead of searing and prevents proper browning.
To the same skillet add 1 tbsp olive oil and heat over medium heat. Add the veggies and cook, stirring occasionally, until crisp tender (about 3 to 5 minutes). Remove veggies from skillet to the plate with the chicken and keep warm.: When you return vegetables to the pan they should sizzle gently, releasing a bright aroma as moisture evaporates and edges soften slightly. Stirring occasionally allows even cooking and slight caramelization without losing snap. Cooking to crisp tender preserves texture so the asparagus and red bell pepper contrast with the creamy sauce later. A typical pitfall is overcooking the vegetables until they are limp and flavorless, so aim for vibrancy.
Add the last tbsp of olive oil to the skillet along with 4 tbsp of butter, over medium heat. Use a spoon or whisk to scrap up any browned bits on the bottom of the skillet (that's extra flavor!). Add the garlic, red pepper flakes and flour, whisk until mixture is thickened and smooth. Slowly pour in the milk and chicken broth, whisking continuously until the liquid is added. Finally add in the parmesan cheese and whisk until smooth and creamy.: The moment you add butter and scrape the pan you will lift concentrated fond into the sauce, which is why that step is so important for depth of flavor. When the garlic becomes aromatic you will smell its sweet, toasty perfume; do not let it brown too dark or it will add bitterness. Whisking the flour into the fats makes a roux, which cooks briefly to remove raw flour taste and thickens gently; if you skip this the sauce can be grainy. Adding milk and chicken broth slowly while whisking prevents lumps and creates a glossy emulsion that will coat the pasta. Stirring in shredded parmesan off heat or on low prevents the cheese from clumping. A common error is adding cold liquids too fast, which causes separation or lumps, so temper gradually.
Add the chicken and veggies back to the pan as well as the cooked pasta. Stir to combine and coat evenly with sauce. Garnish with chopped parsley. Enjoy!: As you fold everything together you will feel the sauce thicken slightly and cling to the penne pasta , creating an even, saucy coating across strands and pieces. The aromas meld, with butter and parmesan rounding out the savory notes while the asparagus and red bell pepper add freshness. Taste for seasoning and adjust with additional salt or pepper if needed. If the sauce feels too thick, a splash of reserved pasta water loosens it while preserving creaminess. Avoid tossing vigorously, which can break delicate pieces of chicken and bruise the vegetables.