Preheat the oven to 350 degrees F. Grease a 9x13 inch casserole dish.Arrange the torn croissants in the baking dish and lay 2 tablespoons of thinly sliced butter over the croissants. Transfer to the oven and bake 5-8 minutes, until the croissants are lightly toasted.Meanwhile, heat the olive oil In a large skillet over medium heat. When the oil shimmers, add the onions and cook until fragrant, about 3-5 minutes. Add the celery and mushroom, and season with salt and pepper. Cook undisturbed for 5 minutes or until golden, stir and continue cooking until the mushrooms have caramelized, 3-5 minutes. Add the butter, garlic, kale, sage, and thyme. Cook, stirring occasionally until the kale has wilted down, about 5 minutes. Remove from the heat and set aside.In a very large mixing bowl, whisk together 2 cups broth and the eggs. Add the toasted croissants and the mushroom/kale mixture + all the butter left in the skillet, gently toss to combine. If the mix seems dry, add additional broth, 1/4 cup at a time until all the bread is moist. Pour the mixture into the casserole dish. Top evenly with cheese. Cover the dish with foil and bake for 35 minutes. Remove the foil and check for dryness, if needed drizzle over extra broth, 1/4 cup at a time, to keep things moist, then continue baking for another 10-15 minutes or until the stuffing is golden on top. Serve warm.