The Gooiest Baked Mac and Cheese

Name: The Gooiest Baked Mac and Cheese
Ingredients: 1 lb. medium shell pasta; 1 Tbsp. Diamond Crystal or 1¾ tsp. Morton kosher salt, plus more; 3 Tbsp. unsalted butter, divided; 1 small onion, grated on the large holes of a box grater; 2½ cups whole milk; 12 oz. American cheese, coarsely grated (about 3 cups); 2 tsp. smoked paprika; ½ tsp. ground mustard; ¼ tsp. cayenne pepper; ¼ tsp. ground turmeric (optional); 12 oz. sharp yellow cheddar, coarsely grated (about 3 cups), divided; 8 oz. Fontina cheese, coarsely grated (about 2½ cups); Freshly ground black pepper