Spread one-third of the three sisters sauce in a 9x13-inch baking dish made from clay or cast iron if you have it.
Mix the cooked wild rice with ground meat, wild onions, and maple syrup. Season with sumac and a pinch of salt. (Wild rice should still have some bite to it - don't overcook it initially since it will continue cooking in the oven.)
Carefully stuff each squash blossom with the rice mixture, gently twisting the petals to seal.
Arrange the stuffed blossoms in the prepared dish, seam-side down.
Pour the remaining three sisters sauce over the blossoms, covering completely.
Scatter the sliced mushrooms and crumbled cheese over the top.
Cover and refrigerate for at least 2 hours or overnight - this helps the flavors meld and the blossoms to soften slightly.
Preheat your oven to 350°F.
Bake covered for 45 minutes, then uncover and bake another 15 minutes until the cheese is golden and the blossoms are tender.
Let rest for 10 minutes before serving - this allows the sauce to thicken slightly.
If you can't find squash blossoms, this filling works beautifully in hollowed-out small summer squash. The three sisters sauce can be made with any winter squash - I often use what I've stored from our fall harvest.
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