Add all marinade ingredients to a mason jar and pulse with an immersion blender until combined yet chunky. Alternatively, use a food processor.
Poke one side of the flank steak with a fork. Season both sides with salt and transfer steak to a zip-top bag. Pour in all of the marinade, massage everything to coat, squeeze out all of the air in the bag and refrigerate 1-4 hours.
When ready to cook, preheat a grill or grill pan to high heat. Remove steak from marinade and brush off any excess liquid. Lay flat on grill pan and cook 5-6 minutes per side or until the thickest part of the steak reads 135F on a meat thermometer. Transfer cooked steak to a board and let rest 10 minutes before slicing.
Wipe down grill pan, add 1 tbsp avocado oil along with all of the bell peppers, jalapeño and onion. Season with a pinch of salt and grill until softened, tossing regularly (approximately 8-10 minutes).
Slice flank steak against the grain and serve with leaves of Boston lettuce, lime wedges, salsa, guacamole and grilled veggies.