Heat oil in medium skillet over medium heat and cook onion, stirring occasionally, until tender, about 3 minutes. Add garlic and cook 30 seconds. Add tomato paste, chili powder, oregano and cumin and cook, stirring frequently, 2 minutes.
Add flour and cook, stirring constantly, until mixture forms a paste. Slowly add water and bring to a boil, stirring constantly. Reduce heat to medium and stir in diced tomatoes, brown sugar and pepper. Bring to a boil over high heat. Reduce heat to low and simmer, stirring occasionally, until sauce starts to thicken, about 15 minutes. Stir in Hellmann's® Real Mayonnaise. Use as a sauce for your favorite enchilada recipe.