Cook the sliced squash until just al dente, then drain well.
In a large bowl, whisk together the egg, sour cream, mayonnaise, heavy whipping cream, and Garlic Jalapeño Rub.
Fold in the shredded cheese, cooked squash, and diced onion until evenly combined.
Transfer the mixture to a casserole dish or disposable aluminum pan.
Combine the melted butter with the crushed Ritz crackers and spread evenly over the casserole.
Preheat the pellet grill to 350°F and cook the casserole for about 50 minutes, or until the filling is set and the topping is golden brown.
Let the casserole rest for 5–10 minutes before serving.
