Preheat the panini grill to High. If you’re making more than 2 sandwiches, preheat your oven to 175F or set it to keep warm.
Mix the olive oil, garlic powder, basil, and black pepper, then brush this mixture lightly over one side of each slice of bread.
Flip the bread slices over so the un-oiled side is facing up, then spread mayo on 2 slices and brown mustard on 2 slices.
Put enough slices of cheese on the mustard side to cover the bread.
Put enough roast beef on the mayo side to cover the bread (we usually put a bit more on there, because, heck, I like meat).
Slice the tomato very thinly into as many slices as you need to create a layer of tomato for each of the sandwiches, then place the tomato on top of the roast beef.
Slice the red onion very thinly into as many slices as you need to create a layer of onion for each of the sandwiches, then place the tomato on top of the tomato slices.
Carefully flip the mustardcheese slice sides over on top of the red onions.
Place the sandwiches on the grill and lower the lid. Press down on the lid for about 30 seconds and then let cook for 4 – 5 minutes.
Slice and serve! You can serve this with a little horse radish, or beef au jus as a dip if you want