2. Lightly butter a 9- by 13-inch casserole dish. Bring a large pot of cold water to a boil and generously salt. Add pasta and boil, stirring occasionally, until almost al dente, about 8 minutes. Drain pasta, transfer to a large bowl, and cool slightly.
3. Melt butter with garlic clove in a medium skillet over medium heat. Add bread crumbs, parsley, and thyme and toss together. Remove garlic clove; set crumbs aside.
4. Toss pasta with cheeses. Fold in bechamel sauce. Mix in tomatoes, red pepper, salt, and pepper. Transfer to prepared baking dish and scatter bread crumbs over top. Bake until sauce bubbles and crumbs crisp and brown, 25 to 30 minutes. Let rest for 10 minutes before serving.
Melt butter in a medium saucepan over medium heat. Stir in flour with a wooden spoon and cook about 2 minutes. Slowly whisk in milk and bring to a boil. Add parsley, thyme, bay leaf, and onion. Reduce heat to a gentle simmer and cook, whisking occasionally, until sauce is thick, about 15 minutes. Remove herbs and onion and season with 1/2 teaspoon salt, nutmeg, and pepper.