1/2 cup plus 2 tablespoons all-purpose flour (spooned and leveled)
4 garlic cloves, minced
6 cups whole milk
10 ounces white mushrooms, trimmed and thinly sliced
1/2 cup oil-packed sun-dried tomatoes, drained and thinly sliced
1 1/2 cups shredded provolone (6 ounces)
1 cup finely grated Parmesan (4 ounces)
Preheat oven to 400. Butter two shallow 2-quart baking dishes. In a large pot of boiling salted water, cook pasta 3 minutes short of al dente; drain pasta, and return to pot.
In a large nonstick skillet, heat oil over medium-high. Season chicken with salt and pepper; cook until opaque throughout, 3 to 5 minutes per side. Halve each piece lengthwise, then thinly slice crosswise.
In a 5-quart Dutch oven or heavy pot, melt butter over medium. Add flour and garlic; cook, whisking, 1 minute. While whisking, gradually add milk; bring to a simmer, whisking frequently. Add mushrooms and tomatoes; cook 1 minute. Off heat, gradually stir in provolone and 1/2 cup Parmesan.
Add chicken and pasta to pot; season with salt and pepper. Divide pasta mixture between baking dishes; sprinkle each with cup Parmesan.
Bake, uncovered, until top is golden and bubbling, about 25 minutes. Let stand 5 minutes before serving.
Prepare through step 4; let cool. Cover tightly with foil, and freeze, up to 3 months.
Preheat oven to 400, and bake (still covered in foil) on a rimmed baking sheet until center is hot, about 1 1/2 hours. Remove foil; bake until golden, about 15 minutes more.