Combine rhubarb, strawberries, honey, 1/3 cup flour, and cinnamon in a large bowl; let rest for 5 to 10 minutes, stirring occasionally. Transfer fruit mixture to a 9x13-inch baking dish.
Mix remaining 1 cup flour, rolled oats, molasses, and nutmeg in another bowl. Cut in butter with a knife or pastry blender until the mixture resembles coarse crumbs. Sprinkle crumb mixture over fruit.
Bake in preheated oven until crumb topping is golden brown, 35 to 40 minutes.
