Pre-heat the oven to 400 F. Place the garlic cloves and olive oil in a small metal bowl and cover with foil, and roast for 20 minutes. The garlic should be very soft and browned. Take out to cool.
In a large pot, place the parsnips and cover with water. Bring to a boil over high heat, then reduce to medium low and simmer until the parsnips are very soft. Test by piercing with a paring knife. Drain well.
Transfer the garlic and oil to a blender or food processor, and add the parsnips and salt. In a food processor, puree completely before adding the milk. In a blender, add the milk and puree. When smooth, transfer to a large piping bag with a large star tip, or just reserve to spread on the pie.
Prepare a 9x13 inch pan or other medium casserole.
For filling: Boil a large pot of water and drop the spinach in, cook for about 2 minutes, then drain. Rinse with cold water, squeeze out, then spread on a thick kitchen towel, roll up, and squeeze until very dry.
In a large pot, heat the olive oil and saute the onions, carrots and sage until golden and tender, about 10 minutes. Sprinkle the flour over the onions in the pan and stir to coat the vegetables with flour and eliminate any lumps. Gradually stir in the milk and stock, and cook over medium heat until thick. Add salt, tamari and pepper and stir. Take off the heat and stir the spinach (and seitan or beans, if using) into the sauce, then spread in the 9x13 inch casserole.
Pipe the parsnips on top of the spinach filling as shown, or place dollops on top and spread them with a spatula. Spritz with a little olive oil, then bake at 400 for 35 minutes. I cranked up the broiler and watched carefully for a few minutes to get my parsnip topping nicely browned.