Melt 1 Tbs. of the butter with the olive oil in a 10-inch straight-sided sauté pan or Dutch oven over medium-low heat. Add the onions and 1/2 tsp. of the salt, cover and cook, stirring occasionally, until the onions are soft and translucent, about 5 min. Uncover, raise the heat to medium, and cook, stirring frequently, until the onions are light golden and shrunken, another 3 to 4 min.
Add the remaining 1 Tbs. butter and the zucchini. Cook, stirring occasionally, until the zucchini is slightly shrunken and almost tender, about 3 min. Add the corn, garlic, and the remaining 1/2 tsp. salt. Cook, stirring frequently and scraping the bottom of the pan with a wooden spoon, until the corn is tender but still slightly toothy to the bite, 3 to 4 min. (It will begin to intensify in color, glisten, and be somewhat shrunken in size, and the bottom of the pan may be slightly brown.) Add the cumin and coriander and cook, stirring, until very fragrant, about 30 seconds.
Remove the pan from the heat, add all but about 1/2 Tbs. of the mint, a good squeeze of lemon, and a few generous grinds of pepper. Stir, let sit 2 min., and stir again, scraping up the brown bits from the bottom of the pan (moisture released from the vegetables as they sit will loosen the bits). Season to taste with more salt, pepper, or lemon. Serve warm, sprinkled with the remaining mint.