Begin by preheating the oven to 350 degrees F. You will be par-cooking the rhubarb to soften it before adding the crumble topping. Add sliced rhubarb and optional strawberries to a large bowl and combine with lemon juice, lemon zest, cornstarch, cane sugar, and vanilla extract. Add into a glass pie pan or 8 x 8 glass dish and set a timer to bake for 20-25 minutes, until some juices have released from the rhubarb.

While rhubarb is softening, begin browning butter by adding it to a skillet over medium heat on the stovetop. Stir consistently to melt and let it take on some color. Actually getting the butter to the "browned" state should take around 5 minutes over medium heat, stirring intermittently. It is finished when it smells nutty and has darkened in color. You will know that it's burned if it starts giving off an acrid smell (try to avoid this by keeping an eye on it).

While butter is browning, combine rolled oats, flour, cinnamon, cardamom (if using), salt, chopped walnuts, and optional maple syrup in a medium-sized bowl.

When butter has finished browning, immediately add to bowl with oat mixture. Use a spatula to stir together to combine. It should have a texture similar to wet sand.

When rhubarb is finished softening in the oven, remove and add crumble topping (spreading across the entire surface in clumps). Place baking dish on a cookie sheet (to catch any overflow juices). Bake for 25-30 more minutes, until bubbling and golden brown on top. Remove from oven and enjoy with vanilla ice cream.