Preheat grill.
Liberally salt and pepper the steak, and let it sit at room temperature while the grill preheats.
Add pineapple, red pepper, red onion, avocado oil and a big pinch of salt and pepper to a bowl. Gently toss to coat, while keeping the red onion slices intact.
Add the steak, and veggies to the grill in an even layer, in batches if necessary. Cook the veggies, turning just once, until softened, about 5-7 minutes per side. Cook the steak on one side for 4-5 minutes, until nicely seared then turn and cook for an additional 3-10 minutes, depending on your desired temperature.
Pull the steak and the veggies from the grill, and let cool for 5 minutes.
Meanwhile, prepare the dressing. Add mustard, pineapple juice, olive oil, white wine vinegar, salt, and pepper to a jar.
Mince half of the cilantro, and add to the jar. Shake up the jar until the dressing is combined. Taste, and adjust seasoning.
Slice the steak, and dice up the veggies.
Assemble the salads: Divide lettuce, avocado, remaining cilantro, sliced steak, onions, pineapple, and bell pepper into four bowls. Drizzle 1-2 tablespoons of dressing over the top. Serve immediately.