Preheat your oven to 400°F and line a baking sheet with parchment paper.
Toss the cubed Yukon Gold potatoes with half of the avocado oil, half of the sea salt, half of the black pepper, and the dried rosemary.
Spread the potatoes in a single layer on the baking sheet and roast for 25-30 minutes, flipping halfway through, until they are crispy and golden.
While the potatoes roast, pat the ribeye steak dry with paper towels and season both sides with the remaining salt and pepper.
Heat a cast-iron skillet over medium-high heat with the remaining avocado oil until the oil is shimmering.
Place the steak in the hot skillet and sear for 3-4 minutes per side for medium-rare, then transfer to a plate to rest for at least 5 minutes.
Lower the heat to medium, add the minced garlic to the same skillet, and sauté for 30 seconds until fragrant.
Add the fresh baby spinach to the skillet and sauté for 1-2 minutes until just wilted.
Slice the steak against the grain and serve immediately alongside the roasted potatoes and sautéed spinach.