Preheat the oven to 375°F (190°C).
In a medium mixing bowl, combine the peas, diced pimentos, water chestnuts, condensed cream of celery soup, and chopped onion.
Stir until everything is evenly coated.
Transfer the mixture to a lightly greased 1½-quart casserole dish.
Sprinkle the shredded cheddar cheese evenly over the top.
In a separate bowl, combine the crushed crackers with the melted margarine until well coated.
Sprinkle the cracker mixture over the cheese.
Bake the Green Pea Casserole for 30–40 minutes, or until the topping is golden brown and the filling is hot and bubbly.
Let rest for 5 minutes before serving.