Heat a stove-top grill pan or griddle with cooking spray and set over medium-high heat to preheat. Season both sides of flank steaks with salt, black pepper, garlic powder, and onion powder. Place steaks on hot pan and cook 5 minutes per side for medium meat. Let steaks rest 10 minutes before slicing crosswise into 1/4-inch thick slices.
While the steak is cooking, heat oil in a large skillet over medium-high heat. Add pearl onions and cook 3 minutes, until golden brown stirring with a spoon.
Add mushrooms and cook 3 minutes, until mushrooms soften.
Add thyme and cook 1 minute, until fragrant.
Add wine and bring to a simmer, for 5 minutes, until sauce reduces to 1/4 cup.
Serve half of the steak over polenta with the onion-mushroom mixture spooned over top.