Sidewinder's Barbecue Sauce

Original from way back in 1980s Houston, Texas - by Fire Chief Rogers with the Sidewinders' Barbecue Team.

Sauces

Makes 3 and 1/2 cups.
3 tablespoons olive oil
1 medium onion, chopped
2 cloves garlic, minced
1 to 3 tablespoons crushed dried red pepper flakes
1 (16 ounce) can tomato puree, salt-free if possible
1 cup cider vinegar, divided
1 tablespoon dry mustard (I use English mustard.)
1/4 teaspoon powdered ginger or 1 teaspoon fresh minced
4 tablespoons brown sugar
1 to 2 tablespoons chili powder
4 large slices lemon

1. In a heavy medium-size saucepan, heat oil over medium heat.  Add onion and red pepper flakes.  Stir and cook until onion is almost soft, then add minced garlic and cook one minute more.  Remove from heat.
2. Add tomato puree and all but 2 tablespoons of the vinegar.  
3. In a small bowl, combine remaining vinegar, mustard, and ginger; stir until it's a paste.  Add to the sauce mixture.
4. Stir in brown sugar, chili powder and lemon slices (don't squeeze - just dump them in).
5. Bring mixture to a boil, reduce heat and simmer covered for 1 hour.  Stir occasionally.
6. Cool to room temperature, then place in a covered jar in the refrigerator.  It will keep for about a month or more.