Place tomatoes, onions, and olive oil in a nonstick pan over medium high. Allow to cook, without stirring, for 4-5 minutes, or until tomatoes start to blister.

Meanwhile, slice polenta into 1/4" thick rounds.

Spray nonstick pan with cooking spray and cook polenta slices in it over medium high until golden and crisp, about 4 minutes per side.

When tomatoes have blistered, stir. Reduce heat to medium. Add garlic, salt, pepper, and vinegar. Allow vinegar to reduce, about 2 minutes.

Top polenta with tomato mixture and serve.