Preheat your oven to 400°F (200°C).
In a large bowl, toss the potato cubes with olive oil, garlic powder, smoked paprika, oregano, salt, and pepper until evenly coated.
Spread the seasoned potatoes in a single layer on a baking sheet. Bake in the preheated oven for 20 minutes.
After 20 minutes, remove the baking sheet from the oven and add the salmon fillets on top of the potatoes. Season the salmon with salt and pepper, and place lemon slices on top of each fillet.
Scatter the fresh spinach around the salmon and potatoes on the baking sheet.
Return the baking sheet to the oven and bake for an additional 15-20 minutes, or until the salmon is cooked through and flakes easily with a fork, and the potatoes are tender.
Remove from the oven, garnish with fresh parsley, and serve warm.