For the almond crumble topping: In a food processor, pulse the almonds until finely ground. Add the flour, sugar and salt. Pulse to incorporate. Add the butter and pulse until the butter is dispersed in walnut-size pieces throughout. Set aside at room temperature.
For the crumble: Mix the rhubarb, strawberries, sugar, cornstarch, lemon zest and lemon juice in a large mixing bowl. Fill four 4-inch ramekins almost to the top. Sprinkle about 2 tablespoons of the almond crumble on the top and place in the oven. Bake until the filling bubbles and the topping is golden brown, about 20 minutes. Let stand for 5 minutes.
For the topping: Using a whisk or whisk attachment and stand mixer, whip the mascarpone on medium-high speed for about 2 minutes, to increase the volume. Reserve and refrigerate until ready for use.
To serve: Once rested, top each ramekin with 2 tablespoons of whipped mascarpone.