Heat a large skillet over medium-high heat. Add the ground beef, diced onion, and minced garlic; cook, breaking up the meat with a wooden spoon, until the beef is no longer pink.
Drain any excess fat from the skillet, return the pan to the stove, pour in the enchilada sauce, stir to combine, and simmer about 10 minutes.
While the beef simmers, heat a second skillet over medium-high heat and add the vegetable oil.
When the oil is hot, fry the corn tortillas one at a time (or a few if the pan fits without overlapping) about 1 minute per side until lightly golden; transfer to a paper towel–lined plate to drain.
Assemble each flat enchilada by scooping some of the meat mixture onto a fried tortilla, sprinkling with shredded Colby Jack cheese, and adding desired toppings such as sliced avocado, diced tomato, sour cream, cilantro, and sliced black olives.
Serve immediately while warm.
