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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.
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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.
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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.
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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.
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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.