Heat olive oil and butter in a skillet over medium heat, and sauté onion until translucent about 5 minutes. Add shrimp, and sauté for about 3 minutes, or until meat is just pink. Remove from heat immediately and season with salt and ground pepper to taste. Set aside.
In a large bowl, combine yellow and red bell peppers, celery, fennel, and hearts of palm. Set aside.
Prepare dressing by adding vinegar, mustard, lemon juice, honey, and celery seed to a food processor. Combine, and while blade is turning, slowly add oil. Season to taste with salt and ground pepper.
Drain off any liquid that has accumulated in shrimp and add them to veggies. Pour in ½ cup of dressing and toss well to combine. Reserve remainder of dressing for another use. Let the salad marinate in the refrigerator for a few hours or overnight. Serve over cold greens and garnish with sliced tomatoes, basil ribbons, and lemon wedges.