This sauce comes together fast, so have all of your ingredients ready to go to prevent burning.
Warm oil in a small saucepan over medium heat. Add flour and cook off, whisking constantly, about 30-60 seconds.
Add in the chili powder, garlic powder, cumin, and oregano; cook, whisking constantly for 45-60 seconds, until fragrant and deepened in color.
Raise the heat to medium-high; add in the tomato sauce and gradually pour in the vegetable stock, whisking constantly to combine until no lumps remain.
Continue cooking until the sauce reaches a simmer. Then reduce heat to medium-low to maintain the simmer (the sauce should continue lightly bubbling) for about 10 minutes, uncovered, until slightly thickened.
Give the sauce a taste and season with salt, as needed. (I usually add 1/2 teaspoon kosher salt, but the saltiness of the sauce will depend on the brand of broth you use.)
Remove from the heat. The sauce will thicken some more as it cools.
Use immediately and enjoy or store for later.
Notes
Make sure you use American-style chili powder (not cayenne pepper) - American chili powder is a blend of spices and is quite mild, whereas international chili powders are usually immensely spicy.