Sauté the garlic in the butter & oil for 2-3 minutes over medium-low heat or until fragrant. Turn off the heat & add in the crushed red pepper flakes & parsley along with a pinch of salt (to taste).
Lay bread slices out on work surface & distribute the garlic butter over all. Sprinkle with the Pecorino. Set aside.
Onion Jam
Sauté the onions in olive oil over medium heat until golden (about 15 minutes). Add the salt, sugar, balsamic vinegar & thyme. Cook for another 10 minutes or until the jam has thickened. Taste & adjust salt, pepper & sugar levels to taste. Pour the jam into a bowl & set aside.
Sandwich Assembly
Preheat panini grill to 350 F.
Divide roast beef into 4 portions & place on 4 of the slices of garlic bread then top with mozzarella, dividing evenly.
Spread the onion jam on top of the mozzarella then place the remaining 4 slices of garlic bread on top of the sandwich. Grill on a panini grill until golden & slightly crispy.
Recipe Notes
• 3/4 cup of sugar will yield a moderately sweet onion jam. Feel free to use more or less sugar to taste.