Place cauliflower, 2 cups of vegetable broth and 2 cups of water in a large saucepan, cover and bring to boil.
Reduce heat to medium low and simmer for approximately 15 minutes or until cauliflower is tender when pricked with a fork.
Drain cauliflower, reserving 1 cup of cooking liquid.
Place cauliflower, 1 cup cooking liquid, ½ cup almond milk, 2 tbsp. olive oil, ½ tbsp. arrowroot powder, garlic, 1 tsp. salt, and ¼ tsp. pepper in a high-speed blender.
Blend on medium-high until mixture is creamy. (I used variable speed 7 on my Vitamix for about 30-40 seconds.)
White Bean Filling
Preheat oven to 400 degrees.
While cauliflower is cooking, add ¼ cup of water to a 5-quart sauté pan.
Add mixed vegetables, broccoli, onion, and chopped celery, cover and cook on medium high. Stir occasionally.
Reduce heat to medium once pan is fully heated and continue to cook for about 15 minutes or until vegetables are tender.
Add navy beans, 1 tsp. thyme and cauliflower sauce to vegetables and stir until well mixed.
Spoon mixture into a 9×13 pan and spread evenly.
Mashed Potato Topping
Before you begin cooking the cauliflower, scrub potatoes and chop into 2-inch chunks. (No need to peel.)
While cauliflower is cooking, add potatoes, 2 cups of vegetable broth and 2 cups of water to a large saucepan, cover and bring to boil.
Reduce heat to medium low and simmer for approximately 20 minutes or until potatoes are tender when pricked with a fork.
Drain potatoes, leaving a small amount of the cooking liquid behind. (Enough to wet the bottom of the saucepan.)
Add ¾ cup almond milk, 1/2 tbsp. olive oil, 1 tsp. dried parsley, ½ tsp. salt and 1/8 tsp. pepper to the potatoes and mash with a potato masher. (Avoid over-mashing or the potatoes will become gummy.)
Use a large spoon and a spatula to spoon and carefully spread the mashed potatoes over the white bean filling in the 9×13 pan. (Be sure to spread mashed potatoes all the way to the pan’s edges to seal in the filling.)
Place pan in an upper center position in preheated oven and bake for approximately 15 to 20 minutes. (Look for tips of mashed potatoes to brown lightly or for filling to bubble up. You may wish to place a baking sheet on the rack below to catch drips.)