1. Heat an outdoor grill or a stovetop grill pan to medium-high.
2. Halve chile lengthwise, remove stem, and scrape out seeds.(For maximum heat, do not remove seeds.) Thinly slice chile and set aside.
3. Stir garlic and olive oil together in a medium microwave-safe bowl, cover loosely with plastic wrap, and microwave on HIGH until garlic is soft and aromatic, about 2 minutes. Stir in chile, lime juice, cilantro, and 1 teaspoon salt to hot oil. Set mojo aside to cool until ready to serve.
4. Brush tuna with olive oil and season generously with about 1/2 teaspoon salt and black pepper, to taste. Grill, turning once, until distinct grill marks are seared into fish, 3 to 5 minutes per side for medium rare or 4 to 6 minutes for medium-well. Let rest for 5 minutes.
5. Slice each tuna steak in half, divide among plates, and drizzle with some of mojo. Serve with green beans and any remaining sauce on the side.
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