- Heat tomato sauce in a saucepan over medium-low until just boiling, about 3 minutes. Turn heat off and cover with lid to keep sauce warm.
- Combine chopped asparagus, red onion, and bell peppers, garlic, soy sauce, 2 teaspoons olive oil, thyme, and black pepper in a medium mixing bowl. Toss to coat vegetables. Heat a medium cast-iron grill pan over medium-high heat for 3 minutes. Add grilled vegetable mixture to pan and grill for 6-7 minutes, stirring occasionally.
- While vegetables cook, slice polenta into ¼-½ inch rounds. Heat 1 teaspoon of olive oil in a medium skillet over medium-high heat for 2 minutes. Add 4-5 slices of polenta and cook for 4 minutes on each side or until lightly browned. Set polenta aside on a serving plate and repeat with remaining polenta slices.
- Spoon 1-2 teaspoons of tomato sauce on each polenta crostini and top with a spoonful of grilled vegetable mixture. Finish with chopped basil and flaky sea salt. Serve warm.