Preheat the oven to 425°.
Dice yellow onion into a fine dice. It doesn’t have to be perfect, but smaller pieces blend with the chicken and cheese to make a nice even filling.
Mince or press garlic.
Heat a skillet on medium high heat. Add olive oil to the pan. Then add the diced onions and minced garlic to the pan. Cook them until they are translucent.
Take cooked chicken and shred or dice into bite-sized pieces. Toss the chicken pieces into the pan with the cooked onions.
Pour one cup of enchilada sauce in with the chicken and onions. Give everything a good stir and bring the sauce up to a simmer. Let it cook for just a minute or two.
Prepare an area to assemble the taquitos. Gather your olive oil, flour tortillas, shredded cheese, and filling.
Pour some olive oil into a small bowl. Then use a basting brush to brush oil onto one side of a flour tortilla.
Turn the tortilla over so that you are filling the side that does not have oil. Sprinkle a layer of shredded cheese onto the tortilla. Then put two spoonfuls of filling on top of the cheese.
Keep the filling in a narrow straight line from one end of the tortilla to the other.
Finally, roll the tortilla tightly around the filling. Place the roll seam side down on a lined baking sheet.
Repeat steps 4 and 5 until you have filled 12 six inch flour tortillas.
Bake on the middle rack of the oven for 15 minutes. After 15 minutes, turn the broiler on for low for 3-5 minutes. This will get them extra crisp.