Add onion in a medium bowl, and toss with vinegar and a pinch of kosher salt. Let sit while you prepare the lentils.
Place lentils in a medium saucepan filled 2/3‘s with water, and bring to a boil over medium-high heat, stirring occasionally. Reduce heat to medium-low, and simmer until the lentils are just tender (about 15 minutes). Start checking for doneness around 13 to 14 minutes, because if you overcook the lentils, they will turn to mush.
Drain the lentils well, then toss with the marinated onion. Stir in olive oil and garlic, and season to taste with salt and pepper. Let lentils cool completely.
Add chopped herbs to the bowl along with half of the tomatoes and feta, tossing to combine. Pour salad onto a serving platter, then top with remaining tomatoes and feta. Finish with a sprinkling of herbs, flaky sea salt and ground black pepper.