Preheat oven to 425 degrees F. Leave the skins on the garlic cloves, but crack them slightly so they won't pop while roasting. Slice the quarter onion into large chunks and separate the pieces. Place garlic cloves, whole Fresno peppers, and onions on a baking sheet and lightly coat with olive oil. Roast for 15 to 20 minutes, checking every 10 minutes, until pepper skins are blistering and browning in spots. Remove from oven.
Remove stems from peppers and garlic skins from garlic, and place the peppers, garlic, and onions in a nonreactive saucepan with the vinegar, sugar, sea salt, and habanero. Bring to a simmer over medium heat, then reduce heat to low and let simmer for 5 to 8 minutes.
Empty all ingredients into a high-powered blender or food processor and blend until ingredients are fully incorporated. Transfer to a glass jar and let cool before sealing. Keep in air-tight container in fridge.