Candice's Skinny Jeans Sliders with Caramelized Onions and Mushrooms

plus 2 tsp extra-virgin olive oil

(about 3 oz.) cremini mushrooms, stemmed and finely chopped

small red onion, finely chopped

garlic clove, finely minced

dried oregano

finely chopped fresh parsley

sea salt

lean ground beef

whole wheat slider rolls

plum tomatoes, thinly sliced