roasting grapes - Place grapes on another quarter sheet pan and toss with 1 teaspoon olive oil. Roast in the oven along with the squash for 10-15 minutes until caramelized. It’s okay if they get slightly charred, but keep an eye on them since the sugar burns easily.
cooking lentils - Prepare the other filling ingredients while the squash and grapes are roasting. Sort through the lentils and pick out any small stones/pebbles or debris (see notes). In a small saucepan, add the black lentils, vegetable stock, ⅛ teaspoon ground black pepper, and ⅛ teaspoon salt. Bring to a boil on medium-high heat, then cover and simmer on low heat for 23 minutes.
sautéing mushrooms - Heat a medium sauté pan on medium-high heat, add 2 teaspoons olive oil and sear the maitake mushrooms. Flip each slice with a spatula until both sides start to brown. Once the mushrooms have browned, season with ⅛ teaspoon salt and then remove from the pan once golden brown with crispy edges. Add diced button mushrooms to the same pan with another ½ teaspoon olive oil and ⅛ teaspoon ground white pepper. Sauté until golden brown while stirring occasionally. Season with ⅛ teaspoon salt, add in the minced garlic, stir and deglaze the pan with sherry.
cooking caramelized onions - In a medium sauté pan (10- to 12-inch works well), add 1 teaspoon olive oil and the sliced onions and cook over medium-low heat while stirring every few minutes. Add ⅛ teaspoon salt and spread out the onions in an even layer each time you stir so they can caramelize evenly. You’ll know to stir when the bottom of the onions start to brown and slightly stick to the pan. Add water when needed to deglaze the pan (see notes) and cook until onions are golden brown, about 25 minutes.
mixing the squash filling - In a medium bowl, combine the caramelized onions, roasted grapes, cooked lentils, and sautéed button mushrooms. Stir until evenly mixed. Scoop about 2 tablespoons of the filling into the cavity of each honeynut squash.