Defrost spinach in warm water. Drain well by squeezing out all the water with a cheese cloth or by hand.
Meanwhile, in a large, heavy-bottomed skillet over medium heat, sauté onions with butter until translucent, about 5 minutes. Add garlic and cook 2 additional minutes. Add spinach and cook until warmed through.
Add cheese, about a cup at a time, until entirely melted and mixed with spinach and onions. You should have a thick, cheesy mass in your pan. Set aside to slightly cool.
In a large Dutch oven, heat oil over medium heat. As oil is heat- ing, place tortillas in a damp, clean dish towel and microwave until warm, about 1 minute. Keep wrapped in the towel until ready to use (make sure your tortillas are warm or they will split). Take 3 tablespoons of the warm cheesy concoction and form into a rope shape that will cover the length of the tortilla. Roll tortilla very tightly. Repeat with remaining tortillas.
Using tongs or a metal slotted spoon, gently lower each taco into the hot oil, pressing the seam of the tortilla down against the bottom of the pan to seal them shut until they begin to brown. Once they are sealed, you can let them cook in the oil until they reach your desired color. This usually take about 4 minutes each.
Drain your rolled tacos on a paper towel or bag. Stack and serve with sour cream and the hot sauce of your choice.