Preheat oven to 350 degrees F. In a medium pot bring water to a boil with a pinch of salt and add shells. Cook until just tender, about 8 minutes. Drain and return to pot without rinsing. Add Velveeta, cream of celery, milk and tuna; stir to combine and keep on low.
In the meantime heat oil in a medium skillet over medium and add carrot, celery and onion. Cook until tender, about 5 to 7 minutes. Add to shells along with peas, salt and pepper; stir to combine. Pour into greased 8 inch round baking dish and top with cracker crumbs.
Bake 35 to 40 minutes, until golden and bubbly. Let rest about 5 minutes before serving.