Bring 8 cups water to a boil in a medium saucepan over high heat. Add the lentils and cook until tender, about 25 minutes. Drain and set aside.
Heat the broiler. Cut each squash in half lengthwise and scoop out the seeds. Cut the squash into 1/2-inch-thick slices, and then into 1-inch segments. Toss the squash with 2 tablespoons of the olive oil and 1 teaspoon salt on a rimmed baking sheet. Add the onions and toss to combine. Spread in a single layer. Broil about 6 inches from the heat until the tips of the squash are charred, about 12 minutes.
Toss the lentils with the remaining 2 tablespoons olive oil and 1 teaspoon salt in a large bowl. Add the vinegar, mint, squash and onions and toss to combine. Serve.
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