Cook garlic in butter in a large frying pan over medium-high heat until fragrant. Stir in mushrooms and yellow onion and sauté until mushrooms are nicely browned, about 8 minutes. Season with 1/4 tsp. salt and 1/8 tsp. pepper. Set aside.
Mix together remaining salt and pepper, turkey, red onion, herbs, and 1/4 cup mayonnaise in a bowl. Shape into 16 patties about 3 in. wide, pressing a dimple into middle of each (this helps them cook evenly).
Heat grill to high (450° to 550°). Brown rolls on cut sides until toasted, about 1 minute. Oil cooking grates, using a wad of oiled paper towels and tongs. Cook patties until grill marks appear on both sides, turning once, 5 to 6 minutes total.
Reheat mushroom mixture over high heat and assemble burgers: about 1 tbsp. cranberry mustard on bottom of each roll, then arugula, patties, and mushroom mixture. Spread tops of rolls with a little mayonnaise and set on burgers.
Note: Nutritional analysis is per 2-slider serving.
We all loved these sliders. They were very moist and flavorful. Prepared them as written but subbed spinach for the arugla because I dont like arugla.The cranberry mustard was very good, but my favorite was the mushroom onion topping. Most of the meal can be made ahead of time and just refrigerated until time to cook, making for a quick weeknight meal. I will be making these again.