Pour chicken broth into a glass measure, adding enough cold water to make 2 cups, then pour into a rice cooker along with brown rice and stir; cook in rice cooker according to manufacturer’s directions, about 40 minutes.
While rice is steaming in the rice cooker, you can get started on the veggies and the sausage.
If you have not already done so, cook sausage, drain, and set aside to cool.
Heat a large skillet over medium high heat, then add oil and butter.
When butter has melted, add sliced zucchini and shredded raw potato, and about ½ tsp salt, a few grinds of black pepper, and sauté until zucchini begins to soften, about 5 minutes.
Add half of the minced sage leaves and the garlic and continue to sauté until the sweet potato starts to also soften and wilt slightly, about another 5 minutes, tasting and adding more salt and pepper if needed, no more than 1/2 tsp; set aside and let cool slightly in a mesh strainer set over a bowl, to catch juices.
At this point, the rice should be almost ready.
Preheat oven to 375°F.
Once rice has cooked fully, allow it to cool slightly, then stir in ½ cup cheese, salt and pepper to taste, and the egg.
Press into the bottom and up the sides of a 9” pie plate or 9" square glass casserole dish.
Sprinkle cooked sausage over the top, then cover with the sautéed zucchini mixture (discarding the juices).
Whisk together eggs, buttermilk,flour, the remaining minced sage leaves, and ½ cup shredded cheese and pour over the top.
If desired, garnish with 3 additional whole sage leaves in the center of the pie (optional).
Bake in preheated oven for about 30 minutes, until filling is set.
Let cool slightly before serving.