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PEEL and finely cut the onion. COMBINE minced meat, mustard, bread crumbs, egg and onion until it's smooth enough to create small hamburgers. Season the meat with pepper and salt.
Use your hands to create small hamburgers. Make sure your arugula is rinsed and then dried with a salad spinner. Use a mushroom brush or paper kitchen towel to clean the mushrooms and cut them into slices.
HEAT the grill pan on high heat and divide a little bit of olive oil onto both sides of the mini hamburgers. GRILL them on both sides for about 2 minutes, or until the grill pattern is visible.
Time depends on the thickness of the burgers. HEAT olive oil in the small skillet and saute the mushroom slices for a few minutes, until golden brown. SEASON with pepper and salt.
Open up the mini hamburger buns and divide a little bit of arugula on the bottom. PLACE the burger and mushrooms on top.
FINISH the burgers with a scoop of cheese and the top of the bun. Enjoy!