Step 1

Build a charcoal fire or heat a gas grill.

Step 2

Place the ground beef in a large bowl and add the mustard, olive oil, thyme, garlic, salt, and pepper. Mix gently with a fork to combine, taking care not to compress the ingredients. Shape the meat into 12 (2-inch) patties of equal size and thickness.

Step 3

When the grill is medium-hot, brush the grill grate with oil to keep the sliders from sticking. Place the sliders on the grill and cook for 4 minutes. Turn the sliders over with a spatula and cook for another 4 to 6 minutes, until medium-rare, or cook longer if you prefer the sliders more well done. For the last 2 minutes of cooking time, place 1/2-ounce Gruyere on the top of each burger and close the grill lid. Remove the sliders to a platter and cover with foil.

Step 4

Slice the buns in half crosswise and toast the halves cut side down on the grill.

Step 5

Divide the baby arugula among the 12 bottom buns, top each with a slider, and finish with a slice of tomato and red onion. Cover with the top of the bun and serve hot with ketchup.