Finely chop all peppers, onion, and garlic; keep texture chunky, not pureed
Heat olive oil over medium heat in a saucepan
Add onion and cook about 3 minutes until softened
Stir in garlic and cook 30 seconds until fragrant
Add chopped peppers to the pan
Stir in vinegar, sugar, salt, oregano, black pepper, red pepper flakes, and jalapeño brine if using
Bring to a gentle simmer
Reduce heat to low and simmer 10–12 minutes, stirring occasionally
Remove from heat and let cool completely
Transfer to a clean glass jar, cover, and refrigerate at least 12 hours before using