Neely's Chicken Salad in Tomato Cups

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