Place a rack in the center of your oven and preheat to 400°F. Spread ½ cup of the sauce in a thin layer in the bottom of a 9x13-inch baking dish.
In a large skillet, heat the oil over medium high. Once it's hot and shimmering, add the ground beef, onion, salt, and pepper. Cook, breaking apart the meat with a wooden spoon, until it's browned and nearly cooked through, about 5 minutes.
Add the garlic, jalapeno, chili powder, cumin, and oregano. Cook for 2 minutes, until the spices and garlic are very fragrant and no pink remains. Taste and adjust the salt and pepper as desired. Remove from the heat. Stir in ¾ cup of the cheese
Coat a rimmed baking sheet with nonstick spray and spread the tortillas into an even layer on the sheet (it's fine if they overlap). Coat tops lightly with nonstick spray. Pop into the oven and bake for 5 minutes, until the tortillas are warm and pliable.
Assemble the enchiladas: Once you can handle the tortillas safely, spread about ¼ cup of the beef filling down center of each tortilla. Roll the tortilla tightly like a sleeping bag around filling, then place seam side down in dish. Repeat, creating two long rows of 7 enchiladas each (snuggle them in close).
Pour the remaining sauce over the top, spread around a bit with the back of a spoon, then sprinkle with the remaining ¾ cup cheese. Bake until the enchiladas are hot and bubbly, 15 to 20 minutes. Sprinkle generously with the green onions and cilantro. Let cool a few minutes before serving with sour cream.
Notes
TO STORE: Store leftovers in an airtight container for 3 to 4 days.
TO FREEZE: Freeze wrapped tightly in casserole dish or transfer smaller portions to an airtight container. Store in the freezer for up to 2 months; thaw in the refrigerator before reheating.
TO REHEAT: Warm in a casserole dish covered with foil in a 350ºF oven until the enchiladas are completely heated through. The microwave also works.