This sauce is what makes these enchiladas special. It's like a Mexican-inspired garlic Alfredo sauce, and it's absolutely worth the extra effort:
Melt the butter in a saucepan over medium heat.
Add the minced garlic and cook until fragrant (about 1 minute).
Sprinkle in the flour and whisk for 2-3 minutes.
Slowly add the chicken broth while whisking to avoid lumps.
Once it thickens, stir in the sour cream and oregano.
Season with salt and pepper to taste.