Bring a large pot of water to a boil over medium-high heat. Add pasta, and cook until al dente. Reserve 1 cup pasta water; drain and set aside.

While pasta cooks, heat 1 tablespoon butter in a large skillet over medium-high heat. Cook beef strips until browned and cooked almost all the way through. Remove beef to a bowl; set aside.

In the same skillet, add remaining tablespoon butter. Once butter melts, add sliced onion. Cook until just beginning to brown; reduce heat to medium-low. Cook onions until tender and almost caramelized, about 10 minutes. Add bell pepper; cook for 3 to 5 minutes. Return beef to skillet; cook to heat through.

Whisk together eggs, evaporated milk, cheeses, salt, and cayenne pepper in a medium sauce pan. Cook sauce over medium heat, whisking frequently, until cheese melt and sauce thickens.

Pour sauce over pasta and stir to combine. Add pasta water until sauce thickens and coat pasta evenly (about ½ to ¾ cup). Top macaroni with beef and vegetables.