Combine lime juice, orange juice, garlic, cilantro, olive oil, beer, and 1 tsp kosher salt in a bowl or Ziploc bag. Add the steak and coat evenly. Marinate 2–4 hours in the refrigerator, up to 6 hours.
Heat oil to 350°F. In a bowl, whisk flour, kosher salt, coarse black pepper, and garlic powder. Add cold water a little at a time, whisking until a smooth, loose batter forms. Add onion strips and toss to evenly coat. Fry in small batches until golden and crisp, 2–3 minutes. Drain and lightly salt with kosher salt.
Heat a skillet over medium-high heat. Char tomatillos, onion, garlic, and serranos. Blend with remaining ingredients.
Heat a plancha over medium-high heat. Lightly oil the surface if needed, then sear the sliced panela until browned on both sides. Remove from the plancha and set aside.
Preheat grill to high heat. Remove the steak from the marinade and pat lightly to remove excess moisture. Immediately before grilling, season with kosher salt. Grill 2–4 minutes per side until medium-rare to medium. Rest 5–10 minutes, then chop.
