Blackberry Glazed Swedish Meatballs

Name: Blackberry Glazed Swedish Meatballs
Ingredients: 1 medium yellow onion, chopped; 2 tablespoon unsalted butter, divided; 1/2 cup (113 grams) heavy cream; 1/4 cup (12 grams) panko bread crumbs; 1/2 pound ground beef; 1/2 pound ground pork; 1 large egg; 2 1/4 teaspoon Diamond Crystal kosher salt, divided; freshly ground black pepper; 2 cup (12 ounces) blackberries, halved; 2 tablespoon brown sugar; 1 tablespoon apple cider vinegar; 1/4 cup (56 grams) water, more as needed; microgreens, for garnish