In work bowl of a food processor, combine garlic and chilies; pulse to form a chunky paste.
Add lime juice, oregano, chili powder, beer and olive oil. Pulse to combine.
Place steak in a shallow dish or large zip-top plastic bag. Add marinade and turn to coat. Cover and refrigerate for at least 3 hours or overnight.
Heat grill to medium-high. Brush excess marinade from steak and season well with salt and pepper. Grill until medium-rare to medium. Set aside and let rest, covered loosely with foil, for 10 minutes before slicing.
Meanwhile, heat a large sauté pan over high heat. Add enough oil to lightly coat pan.
When oil is hot, add peppers and onion; season with salt and pepper. Cook quickly, stirring constantly, until vegetables are just softened and starting to brown. Remove from heat.
Cut steak into thin slices across grain. Top with peppers and onion.