For the squash sauce: Heat 8 tablespoons of the butter in a heavy-bottomed pan over medium heat. When the foaming subsides, add the sage leaves and garlic. Cook until the sage is crisp, the garlic is lightly colored and the butter has a light-brown color and nutty aroma. Add the squash and toss to coat in the butter. Season with salt and pepper. Pour in enough broth to barely cover the squash and bring to a simmer. Cover the pan and cook until the squash is soft enough to crush with a fork, 10 to 15 minutes. Working in batches if necessary, transfer the mixture to a blender and blend until smooth with the remaining 4 tablespoons butter. Season with salt and pepper and strain the sauce through a fine-mesh sieve.
For the fish and vegetables: Heat 1 tablespoon of the oil in a skillet. Season the flesh side of the snapper with salt and pepper. Once oil is hot, add the fish skin side down to the skillet and cook until the skin is golden and crisp, about 5 minutes. Turn the fish over and finish cooking on the other side. Cover and keep warm.
In another skillet, heat the remaining 1 tablespoon oil. Add 2 tablespoons of the butter; once the foaming has subsided, add the squash and cabbage and cook until tender and heated through. Season with salt and pepper.
Reheat 3 ounces of the squash sauce; when very hot, remove from the heat and swirl in the remaining 1/2 tablespoon butter. Place the sauce in the bottom of a bowl, add the squash and cabbage and top with the fish. Combine the pea shoots, 1 teaspoon of the spiced pumpkin seeds and a drizzle of pumpkin seed oil in a bowl and use it to garnish the dish.