To assemble the goat cheese grilled cheese, drizzle two slices of sourdough bread with olive oil. On one slice, place clumps of goat cheese and gently spread to coat. Remember, unlike other cheeses like cheddar cheese, goat cheese has a strong flavor. Use less cheese than you would with traditional grilled cheese, but a generous coating.
Add a layer of sliced tomatoes. Season tomatoes with salt and pepper. Top with caramelized onions.
Wipe out the skillet you used to make the caramelized onions. Add another drizzle of olive oil and a dollop (about 1 tablespoon) of roasted garlic rosemary butter. Once the butter has melted, dip the side of your slice of sour dough that does not have the cheese, tomato and onions. Coat the side of the bread that does not have the olive oil drizzle. Briefly set aside.
Add more butter and olive oil to the pan if necessary. Then, gently lay your sourdough bread with the ingredients onto the melted butter. Top with the remaining slice that you first dunked with butter. Place the second piece of bread on top, butter side up.
Allow your goat cheese grilled cheese to cook for about 3-4 minutes on medium low heat, peeking underneath until it is golden brown. Then, using a spatula, flip the sandwich and cook on the other side, about another 3-4 minutes.
Cut in half diagonally and serve with some optional balsamic glaze.