Preheat oven to 400 degrees F.
Line each of the 2 roasting pans with parchment paper. Place diced potatoes on 1 tray, drizzle with 2 tbsp olive oil, and season with salt, pepper, and 1 tsp or so of any spices you like (such as a combo of garlic powder, chili powder, cumin, oregano, etc.) Roast potatoes in the oven for 25 minutes, then turn and roast an additional 15-20 minutes until potatoes are soft when poked with a fork.
Place sliced peppers and onions on the second pan, drizzle with 1-2 tbsp olive oil, and season with salt and pepper. Roast for 20 minutes, then turn and roast for an additional 5-10 minutes.
When peppers and onions are browned and done, turn off the oven and add the entire bag of spinach to the same tray. Place vegetable pan in the oven for 5 minutes, then turn spinach into hot vegetables until wilted.