In a large skillet over medium heat, melt butter. Add mushrooms and cook until soft, 5 minutes. Add garlic and cook until fragrant, 1 minute more. Add flour and stir to coat mushrooms, then add milk and broth slowly and whisk until no lumps remain. Simmer until sauce is thickened, 5 minutes. Season with salt, pepper, and a pinch of cayenne.
Add pimentos, peas, and chicken and cook until warmed through, 5 minutes.