Preheat oven to 375 degrees F (190 C)
Heat oil in a large skillet over medium heat. Add onions and sauté for 3-5 minutes until softened and lightly browned.
Add garlic and sauté for 30-60 seconds until lightly golden and fragrant.
Now add mushrooms and sauté until softened and browned. About 5-7 minutes.
Add spinach and cook until just wilted, about 1-3 minutes. Sprinkle with salt & pepper if desired. Remove from heat and set aside.
Break up the tofu and add it to a food processor along with the seasonings, 1 tablespoon of plant-based milk, and flour. Process until the mixture is smooth, about 2 minutes. It will be very thick, but if it's too thick and won't smooth out, you can add 1 more tablespoon of non-dairy milk. Just enough to get it moving.
Using a spatula, scrape the mixture out of the container into a medium mixing bowl. Fold in the mushrooms/spinach mixture and combine well.
Place the mixture in the pie crust and spread evenly. Add the cut tomatoes on top and gently press down so they are slightly in the mixture. Bake in the oven for 30-40 minutes until the quiche firms up and is golden on top. Remove from oven and let cool for 10 minutes before cutting.