Preheat the oven to 400 degrees Fahrenheit. Slice the tomatoes in half and place on a baking sheet. Drizzle the olive oil, salt, pepper, and dried rosemary over the tomatoes. Roast for 15 minutes.
While the tomatoes are roasting, begin cooking the chicken. In a large dutch oven, begin warming the olive oil over medium-low heat. Season the chicken breasts with lemon pepper seasoning and place in the dutch oven with the lid on to retain moisture. Flip the chicken consistently during the cooking process to prevent sticking. Once the chicken is cooked, shred into bite sized pieces and set aside, keeping the dutch oven heat on low.
Using a hand held immersion blender, food processor, or vitamix; blend the tomatoes and basil leaves together until pureed. Pour the puree back into the dutch oven.
Turning the heat up to medium-low, whisk in the can of pumpkin puree, half & half, and water until the consistency is smooth. Add the tortellini and the shredded chicken, let simmer for 5-10 minutes.
Serve warm with feta cheese and fresh cracked black pepper sprinkled on top. The longer the soup sits and simmers, the thicker it will get.