Cook the onions. While the lentils are cooking, dice 2 large yellow onions (about 4 cups). Heat 1/4 cup neutral oil in a large frying pan or saucepan over medium-high heat until shimmering. Add the onions and season with 1/2 teaspoon of the kosher salt. Cook, stirring frequently to avoid burning, though some charring will occur in order to get them all thoroughly browned, until very deep golden brown (darker than caramelized onions), about 30 minutes total.