Preheat oven to 375°. Sauté onion in oil. Add rice and sauté 2 to 3 minutes, stirring well. Transfer to a lightly buttered 1 1/2 quart casserole. Dissolve bouillon cubes in boiling water and add sage. Pour over rice. Place spinach on top of rice. Bake covered at 375° for about 45 minutes or until liquid is absorbed. When done, stir spinach into rice.