Preheat your oven to 400°F (200°C) and line a small baking sheet with parchment paper.
Dice the red potato into small half-inch cubes to ensure they roast quickly and evenly.
In a bowl, toss the potato cubes with half of the olive oil, sea salt, black pepper, and garlic powder until well coated.
Spread the potatoes in a single layer on the baking sheet and roast for 20 minutes until they are golden brown and tender.
While the potatoes roast, drain the tinned mackerel and gently pat the fillets dry with a paper towel to remove excess moisture.
Heat the remaining olive oil in a non-stick skillet over medium-high heat.
Place the mackerel fillets in the hot pan and sear for 2 minutes per side until the edges become crispy and slightly caramelized.
Arrange the fresh baby spinach on a serving plate as a base.
Top the spinach with the hot roasted potatoes and the seared mackerel fillets.
Finish the dish with a drizzle of fresh lemon juice and a sprinkle of chopped parsley for a bright, clean finish.