The combination of avocado and grapefruit makes a refreshing salad that's best served at the end of the meal.
Cut avocados in half lengthwise, working around pit; twist to open. Remove pits; peel. Cut avocados into 1-inch pieces; toss in bowl with 1/4 cup lime juice.
Using a sharp knife, cut away peel, pith, and outer membranes from grapefruits. Slice flesh crosswise into 1/4-inch-thick rounds; place in bowl.
Make vinaigrette: Combine remaining 1/4 cup lime juice and honey in small bowl. Whisk in olive oil and red-pepper flakes; season with salt and pepper.
Place lettuce in serving bowl. Season with salt and pepper; drizzle with a quarter of the vinaigrette. Toss well.
Drizzle avocado pieces with three-quarters of remaining vinaigrette. Season with salt and pepper; toss well. Scatter avocado and grapefruit slices over greens; drizzle salad with remaining vinaigrette.