First get your potatoes roasting. Start by preheating your oven to 400 degrees F and lining a large baking sheet with parchment paper.
Combine olive oil with salt, garlic powder, paprika, and pepper. I like to mix it all up and pour it into a ziploc bag, add the cubed potatoes, seal it up, and shake well to coat the potatoes with oil and spices. You could also add everything into a large mixing bowl and stir until coated.
Spread potatoes out on baking sheet and roast for 30-40 minutes, until tender and slightly browned.
While the potatoes are roasting, prep your Brussels sprouts in much the same way you did the potatoes--line a baking sheet with parchment paper, mix the sprouts with the lemon juice, oil, salt, pepper, and pine nuts (either in a ziploc bag or a mixing bowl) and spread the coated sprouts out on the baking sheet. Set aside until potatoes have only about 15 minutes of cooking time remaining, then scoot them over and slide your Brussels sprouts in next to them. Let them roast for the remaining 15 minutes, or until they turn a lovely golden-brown.
As soon as your potatoes are in the oven and your Brussels sprouts are prepped and ready to go, get to work on the stir-fry. Melt the coconut oil in a large skillet and add onions and garlic. Saute until onions are soft and garlic is just beginning to brown.
Add mushrooms and seitan sausage and stir-fry until heated through, adding salt and pepper as you go (to taste).
Add fresh raw spinach leaves and stir-fry just until wilted. then remove skillet from heat.
Toss roasted potatoes, roasted Brussels sprouts, and stir-fry mixture in skillet or large pot/bowl. Serve hot!