Soak the bread in the buttermilk. While the bread is soaking, sauté the celery in olive oil for four minutes, then add the pepper, onion, and garlic. Season with kosher salt and cook three more minutes. Let cool.
Combine the beef, sausage, cooled vegetables, roasted red peppers, parsley, eggs, Italian seasoning, and soaked bread by hand until just mixed.
Refrigerate one hour. While the mix is in the refrigerator, preheat grill for two-zone cooking at 235°F.
Shape mixture into two loaves (2" x 6" x 10") on parchment paper trimmed closely around the loaves, then transfer them to a wire cooling rack.
Place the racks on grill over indirect heat. Close lid and allow to cook until internal temp reaches 150°F, about 1 hour.
While loaves are cooking, whisk sauce ingredients together in a saucepan and warm over low heat.
Glaze the loaves, increase grill to 350°F, and cook for an additional 15 minutes.