Preheat the oven to 425° F.
Prepare the filling: In a medium saucepan, combine hot water, granulated sugar, and cream of tartar. Bring to a boil over medium-high heat and boil for 1 minute.
1 cup hot water, 1 ½ cups granulated sugar, 2 teaspoons cream of tartar
Add lemon juice and quartered Ritz crackers. Do NOT stir. Let the mixture boil gently for 2 minutes, then remove from heat.
juice of ½ lemon, 24 Ritz crackers
Pour the mixture into the unbaked pie crust. Sprinkle with ½ teaspoon cinnamon and dot with small pieces of butter.
1 unbaked 9-inch pie crust, ½ teaspoon ground cinnamon, 1 Tablespoon unsalted butter
Make the topping: In a small bowl, combine crushed Ritz crackers, brown sugar, and cinnamon. Stir in melted butter until evenly coated and crumbly.
1 cup crushed Ritz crackers, ⅓ cup brown sugar, ½ teaspoon ground cinnamon, ⅓ cup melted butter
Sprinkle the topping evenly over the pie filling.
Bake for 30–35 minutes, or until the crust and topping are golden brown and the filling is bubbling.
Cool completely at room temperature before slicing. This helps the syrup set for clean slices.