4 ounces grated Cheddar
2 ounces finely grated Parmigiano-Reggiano or other hard cheese
1 tablespoon unsalted butter
1 small yellow onion, chopped
6 ounces cremini or white button mushrooms, sliced
3 tablespoons unbleached all-purpose flour
3 cups low-fat or fat-free milk
1 tablespoon Dijon mustard
1 tablespoon minced tarragon leaves or 2 teaspoons dried tarragon
½ teaspoon salt
½ teaspoon freshly ground black pepper
8 ounces dried whole-wheat pasta shells (not the large ones for stuffing), cooked and drained according to the package instructions
4 cups small broccoli florets, cooked in boiling water for 1 minute (broccoli can be added to the pasta during the last minute of cooking, then drained with the pasta in a colander)