Step 1: Preheat your oven to 350°F. Crush graham crackers into fine crumbs using a food processor. Mix the crumbs in a bowl with oats, brown sugar, and flour. Add cubed butter and blend until the mixture forms small clumps. Spread it evenly on a baking sheet and bake until golden and crisp, about 20 minutes, stirring halfway through. Let it cool.
Step 2: Whip the heavy cream and powdered sugar in a large bowl until stiff peaks form. Set aside about 1 cup for topping.
Step 3: In another bowl, mix condensed milk, lime juice, vanilla, and a drop of food coloring (if using). Gently fold this mixture into the whipped cream until just combined.
Step 4: Assemble the dessert by layering about 1/4 cup of crumble into 4 glasses. Add two spoonfuls of mousse, then repeat the layers of crumble and mousse.
Step 5: Use the reserved whipped cream to pipe onto the mousse in each glass. Sprinkle any leftover crumble on top and add a lime wedge for garnish.
Step 6: Keep the desserts chilled until you’re ready to enjoy them.