Sprinkle chicken with a pinch each of the Italian seasoning, salt and pepper. In a large deep non-stick skillet, melt butter over medium heat.Brown chicken well, turning once, about 10 minutes; remove to plate.
Increase heat to medium-high; add mushrooms, onion, garlic and remaining seasoning to skillet. Cook, stirring occasionally, until mushrooms are browned and most of the liquid has evaporated, about 6 minutes.
Stir flour into skillet; cook, stirring for 1 minute. Stirring constantly, slowly pour in milk. Stir in pasta and cooked chicken with accumulated juices, tucking into liquid. Bring to simmer, stirring often. Adjust heat as necessary to maintain simmer; cook, stirring often until pasta is almost tender and sauce is still runny, about 10 minutes. (If sauce is reducing too quickly, stir in enough water to keep it saucy and pasta submerged, adding about 2 tbsp water at a time.)
Stir in peas and lemon juice; cook until pasta is al dente and sauce is desired consistency, about 3 minutes.