Bring a large pot of lightly salted water to a boil. Add spinach and cook until wilted, about 1-2 minutes. Drain the spinach into a colander and rinse with cold water. Squeeze as much water out of the cooked spinach as possible.
Add butter, garlic, and red onion to a skillet over medium heat. Saute until the onions are translucent, about 5 minutes.
Add milk, flour, cheese, and cumin to the skillet. Cook stirring occasionally until the cheese melts and the mixture is smooth.
Add salt and pepper and stir to combine.
Fold in the spinach and simmer over medium heat until the mixture thickens.