Preheat your oven to 400°F and line a large sheet pan with parchment paper for easy cleanup.
Peel and dice the sweet potato into uniform 1/2-inch cubes to ensure they cook evenly.
Place the sweet potato cubes on the sheet pan and toss with half of the olive oil, sea salt, and black pepper.
Roast the sweet potatoes for 15 minutes until they just begin to soften and develop golden edges.
Remove the pan from the oven, move the potatoes to one side, and place the salmon fillet on the other side.
Brush the salmon with the remaining olive oil and season with garlic powder plus the remaining salt and pepper.
Return the pan to the oven for 10-12 minutes until the salmon is opaque and flakes easily with a fork.
During the final 2 minutes of roasting, pile the fresh spinach onto the pan to wilt slightly in the residual heat.
Finish the dish by drizzling fresh lemon juice over the salmon and serve immediately.