Make chimichurri dressing: In a food processor, whirl all ingredients except for chipotles until minced. Transfer to a small bowl and stir in chipotles.
Prepare steak and salad: Rub steak and zucchini with oil, then sprinkle with 1 tsp. each salt and pepper. Let stand at room temperature 30 minutes. Meanwhile, heat a grill to high (450° to 550°).
Grill steak, turning once, 6 to 8 minutes total for medium-rare. Transfer to a board, tent loosely with foil, and let rest 5 minutes. Meanwhile, grill zucchini and bell peppers, turning occasionally, until grill marks appear on zucchini (4 minutes) and bell peppers are blackened (about 10 minutes).
Put bell peppers in a bowl and cover. When cool enough to handle, peel, stem, seed, and dice. Slice steak diagonally across the grain into thin strips, then cut pieces in half. Cut zucchini in half on a diagonal.
Toss lettuce in a bowl with half the dressing, then divide among plates. Divide zucchini, bell peppers, steak, and avocado slices among plates, then drizzle remaining chimichurri on top. Garnish with radishes and season with more salt and pepper to taste.
*Silverskin is the thin, shiny connective tissue on the outside of the meat that gets stringy when it cooks. Remove it with a sharp knife, or ask a butcher to do it.