Preheat your oven to 400°F and line a large baking sheet with parchment paper for easy cleanup.
Scrub the potato and dice it into 1/2-inch cubes, then toss them in a bowl with half of the olive oil, sea salt, black pepper, and garlic powder.
Spread the potato cubes in a single layer on the baking sheet and roast for 20-25 minutes until they are golden and crispy.
While the potatoes roast, slice the sirloin steak into thin strips across the grain and season them with the dried oregano.
Heat the remaining olive oil in a large cast-iron skillet over medium-high heat until shimmering.
Add the sliced onions and bell peppers to the skillet and sauté for 5 minutes until they are softened and show slight char marks.
Push the vegetables to the edges of the pan, add the steak strips to the center, and sear for 2-3 minutes per side until browned.
Add the baby spinach to the skillet and stir for about 1 minute until the leaves are just wilted and bright green.
Assemble the power bowl by placing the crispy roasted potatoes at the bottom and topping them with the savory steak and vegetable mixture.