Step 1

Preheat oven to 400°F. Bring a large saucepan of water to a boil.

Step 2

Cook macaroni 1 minute less than the package directions for al dente. Drain and set aside.

Step 3

Meanwhile, heat 1 tablespoon oil in a large ovenproof skillet over medium-high heat. Add onion and cook, stirring, until starting to brown, about 2 minutes. Add zucchini (and/or summer squash) and 1/4 teaspoon each salt and pepper; cook, stirring, until starting to soften, 2 to 3 minutes. Transfer to a bowl and set aside.

Step 4

Reduce heat to medium and melt butter in the skillet. Sprinkle with flour and whisk until fragrant, 1 to 2 minutes. Whisk milk, dry mustard, cayenne and the remaining 1/4 teaspoon each salt and pepper in a small bowl. Slowly pour the milk mixture into the skillet, whisking constantly. Cook, stirring, until the sauce is thick enough to coat the back of a spoon, 3 to 5 minutes.

Step 5

Remove from heat and stir in cheese and pimientos. Using a slotted spoon, add the reserved vegetables to the skillet along with the pasta. Stir to coat.

Step 6

Mix panko, garlic powder and the remaining 1 tablespoon oil in a small bowl. Sprinkle over the casserole. Transfer to the oven and bake until the sauce is bubbling and the topping is golden, about 15 minutes.