Chicken Salad in Tomato Cups

I found this on the Food Network site (thanks Neely!). This dish is so perfect for a ladies lunch (which is where I served it). My dad had the leftovers and he loved it too. I couldn't find tarragon but it tasted fine without it. I really loved the texture that the slivered almonds added. So making this again!

Ingredients
1 rotisserie chicken, skinless and meat shredded or diced
1/2 small red onion, diced
1 shallot, chopped
2 tablespoons freshly chopped tarragon leaves
1 celery rib, chopped
1 cup slivered almonds
2 tablespoons white wine vinegar
3/4 cup mayonnaise
3 cups halved seedless red grapes
Kosher salt and freshly ground black pepper
6 large beefsteak tomatoes
1 head butter lettuce

Directions
Mix first 10 ingredients in a large bowl.
Core tops of tomatoes with a paring knife. Using a melon baller, scoop out the insides of the tomatoes. Lay a leaf of lettuce in each tomato so it peeks out the top. Fill the tomatoes with a large scoop of the chicken salad and serve.