Killer Mac and Cheese with Bacon

Name: Killer Mac and Cheese with Bacon
Ingredients: Extra-virgin olive oil; 6 slices bacon, cut crosswise into 1/2-inch pieces; 3 tablespoons unsalted butter; 1 onion, cut into 1/4 inch dice; Kosher salt; 1/2 cup all-purpose flour; 1 quart whole milk, plus more as needed; 1/4 cup Dijon mustard; Tabasco or other hot sauce; 1 pound medium shells or other short pasta; 2 cups grated Cheddar cheese (about 8 ounces); 2 cups grated fontina cheese (about 8 ounces); 1 cup grated Parmesan cheese (about 4 ounces)