Heat oil in a medium saucepan over medium-high heat. Add diced onion and cook until just browned, about 5 minutes. Stir in garlic, chili powder, cumin, salt and oregano and cook for about 30 seconds, stirring constantly. Sauteing your spices for a few seconds will give your sauce a fuller flavor.
Add in the tomato sauce and chicken stock. Bring mixture to a simmer and cook for approximately 8 to 10 minutes, stirring occasionally.
Remove from heat, and strain mixture through a fine mesh strainer, pressing on the onion to extract the sauce. If I am making enchiladas or something similar, I'll add the reserved onions to my filling. You may discard the onion if you wish.
Keep refrigerated for up to one week. Makes approximately 2 cups.
Notes
adapted from Cook's Illustrated Cookbook
Recipe by Kitchen Meets Girl at https://kitchenmeetsgirl.com/homemade-enchilada-sauce/