Preheat oven to 350°F. Grease a 2-quart baking dish; set aside.
In a large skillet or Dutch oven, melt butter over medium-high heat. Add onion and celery; cook, stirring occasionally, until the onion is translucent, about 5 minutes. Add garlic; cook until fragrant, about 30 seconds.
Reduce the heat to a very low flame. Stir in soup, mayonnaise, sour cream, chives, parsley, thyme, 1 cup of the cheese, and milk. Cook, stirring constantly, until the cheese melts, about 2 minutes. Remove from heat.
Add cooked rice, chicken, green beans, and pimentos; stir to combine. Taste and season with salt and pepper, if necessary.
Transfer mixture to the prepared baking dish. Top with remaining ½ cup of cheese and crushed potato chips.
Bake, uncovered, for about 25 minutes, or until the filling is hot and bubbly and the topping is lightly browned.