Heat oven to 375 degrees. Butter a 3-quart casserole dish; set aside.
Place breadcrumbs in a medium bowl. Toss with olive oil, parmesan cheese and cayenne. Set aside.
Heat about 2 cups of the milk in the microwave (you don’t have to be precise) until almost boiling.
Melt butter in a high-sided skillet over medium heat. When butter bubbles, add flour. Cook, whisking, about 1 minute.
While whisking, pour in a small amount of the hot milk. Once absorbed into flour, pour in more. By the time you’re done with the hot milk you should have a very thick sauce. Slowly pour in the rest of the milk, whisking the entire time. Continue cooking, whisking constantly, until the mixture bubbles and becomes thick.
Remove pan from heat. Stir in nutmeg, mustard, black pepper, cheddar cheese, and salt to taste (about 2 teaspoons) set cheese sauce aside.
Cook pasta in salted water 2 to 3 minutes less than manufacturer's directions, until the outside of pasta is cooked and the inside is underdone. (Different brands of macaroni cook at different rates; be sure to read the instructions.) Reserve about ¼ cup of the cooking water.
Drain pasta then stir into the cheese sauce. Stir in the cans of ortega peppers. If the mixture is too thick, add ½ the reserved pasta water. It should be like a thick alfredo sauce. If it’s still too thick, stir in the rest of the pasta water.
Pour mixture into prepared dish, layering in the melty cheese.
Slice the tomatoes. Arrange in one layer over the macaroni.
Sprinkle breadcrumbs evenly over top. Bake until browned on top, about 30 minutes. Wait at least 5 minutes before digging in