Preheat oven to 350°F and grease a 9-by-13-inch casserole dish.
Sauté onion in olive oil. Add ground beef and taco seasoning; cook until browned.
Melt butter in the skillet, whisk in flour, then gradually whisk in beef broth. Season with salt, cumin, and chili powder; simmer until thickened.
Spread a thin layer of sauce in the baking dish. Stir 1/2 cup Monterey Jack and Cheddar into the beef.
Warm tortillas, fill each with beef, roll tightly, and place seam-side down in the dish.
Pour sauce over the enchiladas and top with remaining cheese. Cover and bake 20 minutes; uncover and bake 15 minutes more until bubbly.