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1
Preheat oven to 350 degrees F (175 degrees C). Grease a casserole dish.
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2
Bring a large pot of lightly salted water to a boil. Cook elbow macaroni in the boiling water, stirring occasionally until tender yet firm to the bite, about 8 minutes. Add baby spinach to the pot in the last minute of cooking; drain.
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3
Melt 2 tablespoons butter in a saucepan over medium heat; cook and stir onion until translucent, about 3 minutes. Stir garlic into onion mixture and cook until fragrant, about 1 minute. Whisk 2 tablespoons pesto, flour, black pepper, and salt into onion mixture until smooth; whisk milk in gradually. Cook and stir milk mixture until slightly thickened, about 2 minutes; stir in 1 1/2 cups Cheddar cheese, 1/2 cup Parmesan cheese, and 1/2 cup Asiago cheese; remove saucepan from heat and stir until cheeses are melted, about 1 minute.
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4
Stir cheese mixture into macaroni mixture; pour into prepared casserole dish. Cover dish with aluminum foil.
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5
Bake in the preheated oven for 25 minutes. Remove casserole from oven; increase oven temperature to 450 degrees F (230 degrees C).
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6
Melt remaining 1 tablespoon butter in a small saucepan; stir in remaining 2 tablespoons pesto, 1/4 cup Cheddar cheese, 1/4 cup Parmesan cheese, 1/4 cup Asiago cheese, and bread cubes. Remove casserole dish from oven, uncover, and top with bread mixture.
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7
Bake casserole with topping in the preheated oven, uncovered, until bubbly and golden, about 10 minutes.
