Cabrito Butter Burger

Name: Cabrito Butter Burger
Ingredients: 1 stick salted butter, at room temperature; 1 Tablespoon minced parsley; 1 Tablespoon finely minced shallots; 1 Tablespoon minced tarragon; 1/2 Teaspoon ground white pepper; 2 Tablespoons lemon juice; 1/2 Teaspoon Coleman's dry ground mustard; 3 onions, peeled, bloom end left on but trimmed so that the onion pieces stay together when cut; 3 Tablespoons olive oil; Salt and pepper, to taste; 15 Roma tomatoes; 2 Tablespoons olive oil; 20 Ounces ground goat, divided into 4 patties, seasoned with salt and pepper; 4 pretzel buns, buttered and griddled crisp; Roasted tomatoes, grilled onions, and seasoned butter, for serving