Preheat an oven to 375 F. Grease a 9x13 baking dish with the softened butter and set aside.
Prep the cheeses. Use a box grater to shred the cheddar, fontina, and smoked gouda. Mix them all together so that the cheeses are evenly mixed. Set aside. Cut the velveeta into small cubes and set aside.
Cook the pasta. Bring a pot of salted water to a boil and add the bouillon, if using. Once the water is at a rolling boil, add the pasta to the water and stir well. Cook until al dente, usually 2-3 minutes shy of the package instructions.
Mix the wet ingredients. While the pasta is cooking, whisk together the eggs and the evaporated milk until very smooth. This is important! You don't want any visible un-mixed egg so whisk until the egg is totally incorporated. Then, whisk in the spices, salt, pepper, and heavy cream until smooth.
Drain and mix the pasta. Drain the pasta in a colander and then immediately transfer to a large mixing bowl while still warm. Stir in the sour cream. Add 1/3 of the cheese to the mixing bowl and stir while drizzling in half of the milk mixture. The goal is mix until the cheese is starting to melt.
Layer the mac and cheese. Pour half of the macaroni mixture into the prepared dish. Top with another 1/3 of the cheese and the cubed velveeta. Top with the rest of the macaroni mixture and pour the remaining milk mixture over the top. Top with the remaining cheese.
Bake. Transfer to the oven and bake for 20 minutes, top with the fried onions and bake 5 minutes more until the top is browned and the mac and cheese is hot and bubbling. Remove it from the oven and let it rest 15 minutes before serving.