Heat a medium non-stick pan over medium-high heat. Add 2 tsp. olive oil, peppers, potatoes, 1/4 tsp. salt, and 1/4 tsp. pepper to hot pan and cook, stirring occasionally, until vegetables begin to brown, 4-7 minutes. Transfer vegetables to second prepared baking sheet and spread into a single layer. Roast until vegetables are brown and tender, 15-20 minutes. Reserve pan; no need to wipe clean. While vegetables cook, cook spinach.