Combine the lemongrass, garlic, shallots, jalapeños, brown sugar, lime juice, and olive oil in the bowl of a food processor and blend until smooth.
Place the meat in a large baking dish and pour the marinade over the top, turning evenly to coat. Cover with plastic wrap and let marinate in the refrigerator for at least 2 hours, or overnight.
Place a grill pan over medium-high heat. Lift the meat from the marinade and place on the grill. Cook for 4 to 5 minutes per side, until cooked through. Transfer the meat to a cutting board and let rest for 5 minutes. Slice the steak against the grain and serve.