Make your croissants the day before and let them sit out overnight. When ready to make recipe, preheat oven to 350F/177C. Lightly oil a 9x13 casserole dish.
Arrange the torn croissants in the baking dish and cover with 2 tablespoons vegan butter, thinly sliced. Transfer to oven and back for 5-8 minutes, until croissants are lightly toasted.
Meanwhile, heat olive oil in large skillet over medium heat. When oil begins to shimmer, add garlic and sage and cook until fragrant, being careful not to burn the garlic. Add the onions and simmer until translucent. Add a dab of vegan butter and continue to cook the onions until they caramelize, about 10 minutes.
Add celery, thyme, kale, sea salt and pepper. Stir well and simmer over medium heat until kale starts to wilt.
Add mushrooms and stir until they begin to "sweat".
In large mixing bowl, whisk together vegan broth and flax eggs until well combined.
Add croissants to broth mixture and stir well.
Add the mushroom/kale mixture and all the butter left in skillet. Toss gently. If the mixture seems to dry, simply add a bit more vegetable broth and stir.
Scoop mixture into casserole dish. Top evenly with vegan cheese.
Cover dish with foil and bake for 20 minutes. Remove foil and check for dryness. Drizzle more broth on top if needed. Continue baking for another 10 minutes or so until stuffing edges are golden brown.
Remove from oven and allow to cool for 5 minutes. Serve piping hot right out of oven.