Preheat oven to 350 degrees F (175 degrees C). Spread almonds on a baking sheet.
Bake in the preheated oven until toasted, about 5 minutes.
Brush acorn squash with walnut oil and sprinkle with pepper. Place cut-side down in a baking pan. Pour in 1/4 cup water.
Bake squash in the preheated oven until mostly tender, about 30 minutes. Drain water and turn squash over. Cut a shallow X into the flesh of each half.
Mix toasted almonds, spinach, ricotta cheese, brown rice, 1/4 cup Parmesan cheese, garlic powder, nutmeg, and sage together in a bowl. Stir in half-and-half gently until filling is moist but still dry enough to hold its shape. Season with salt and pepper.
Stuff squash halves with filling, pressing gently until topped by 1 inch. Sprinkle 2 tablespoons Parmesan cheese, salt, and pepper on top.