Bring a large pot of water to a boil and cook pasta according to package directions, usually about 6-8 minutes.
While the pasta is cooking, sauté onions and garlic in butter on low-medium heat in a sauté pan for about 4-5 minutes. Remove from heat.
In a blender or food processor, combine cottage cheese, 1.5 cups shredded cheddar cheese, milk, salt, black pepper, and the cooked onions and garlic. Blend on high until completely smooth and creamy.
When the pasta is finished cooking, drain noodles and return back to the pan but do not return to heat.
Let noodles cool for 3-4 minutes before pouring sauce over the noodles and then stirring to combine. *Note: To prevent the sauce from separating, it's best to add the sauce to noodles that are not scalding hot. If the noodles aren't warm enough after adding the sauce, return the pan to low heat and heat the noodles and sauce until warm enough to serve.
Next, transfer the mac and cheese mixture to a casserole dish and then top with remaining 1/2 cup of shredded cheddar cheese. Broil on low until cheese is fully melted and serve! *Note: If you'd like to skip the oven, you can also top the mac and cheese mixture with 1/2 cup of shredded cheddar and cover with lid until cheese melts.