Sweet and mild pimiento peppers add a little zest to this skillet mac and cheese, while zucchini boosts the veggie count. Here we boil the pasta a little less than usual, until just shy of al dente, so it doesn't overcook in the oven.
Preheat oven to 400°F. Bring a large saucepan of water to a boil.
Cook macaroni 1 minute less than the package directions for al dente. Drain and set aside.
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.
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.
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.
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.
Small pimiento peppers are sometimes dried and ground to make paprika. Find jars of them near the pickles and olives at the grocery store.