Place the seasoned ribeye on a wire rack set over a baking sheet.
Roast for 20–30 minutes, until internal temp is about 90–95°F (32–35°C) for rare, or 100–105°F (38–41°C) for medium-rare. Remove from oven and let rest briefly.
In a large bowl, combine: Watermelon, cucumber, jalapeño, garlic, mint, lemon juice, honey, olive oil, salt, and pepper.
Mix well and refrigerate for at least 1 hour (or overnight for best flavor). Wait to add feta until just before serving.
Heat a cast-iron skillet over high heat until nearly smoking.
Add 1 tbsp oil and sear the steak for 1–1.5 minutes per side, until a golden-brown crust forms. Optional: Add butter, smashed garlic, and herbs, basting with the melted butter for 1 minute.
Transfer steak to a cutting board or plate. Rest for 5–10 minutes. Slice against the grain and serve immediately.
Before serving, gently fold the feta chunks into the watermelon salad. Top with a sprinkle of pistachios for crunch.
Plate slices of ribeye steak alongside a chilled scoop of watermelon feta salad.