Dump your pasta into a large dutch oven or oven-safe pot. If you don’t have a large oven-safe pot, that’s fine-- but then you must transferring the mac and cheese to a casserole dish later.
Pour cold water into the pot until the water level is about half an inch above the pasta (pasta will be fully submerged). Simmer on medium-low heat until the pasta is fully cooked. Make sure to stir from time to time to prevent the pasta from sticking to the bottom!
While the pasta cooks for 8-10 minutes, preheat your oven to 500 degrees or a low broil. Also, this is a great time to shred your cheeses with a box grater. I realized I didn’t have one and had to chop my cheese by hand…:/
Take a bite of a pasta and check if it is al dente, or still has an uncooked center. Once al dente, reduce the heat to the lowest setting and mix in the cream of mushroom soup and evaporated milk. Once the mixture begins to produce small bubbles, add your shredded cheeses. Stir well until combined, then cover with the lid and let it sit on low heat for 5 minutes. The residual heat inside the pot + accumulating steam will melt the cheeses down without scalding the milky base.
After the cheese is thoroughly melted, add the mustard, garlic powder, umami seasoning, and 5 Tbsp of butter. Stir well and taste for any needed salt and pepper. The sauce should be silky and smooth (and flavor-packed as hell). Keep covered and remove from heat.
The sauce should be one level thinner than your desired consistency. If the sauce is too thick, you can add 1/4 cups of milk at a time and mix in until you reach your desired creaminess. Keep in mind it will thicken more once it cools down, so I like to keep mine on the saucy side.
To make the crispy topping: roughly crush 1 cup of the fried onions with the back of a wooden spatula or rolling pin. They should still be crumby, not crushed to a dust. Melt the 5 remaining Tbsp of butter in a small bowl. Mix the breadcrumbs and onions with the butter. They’re about to get all fried and crispy in the oven!
If you’re using an oven safe pot, go ahead and evenly top the mac and cheese with the onion crumb mixture. If you’re not, transfer the mac and cheese to a casserole dish and then sprinkle the breadcrumb mixture on top.
Pop this in the oven for 3-5 minutes! Keep your eye on it to make sure the crumbs don’t burn. Once you’ve taken it out of the oven, let it rest for at least 5-10 minutes before serving and taking Lactaid pills. Many many Lactaid pills.