Prepare the Puree: Reserve 1-2 stalks of rhubarb for poaching. Chop the remaining rhubarb and simmer with the strawberries, lemon juice, sugar, and water for 10-15 minutes until broken down. Blend until smooth and set aside.
Infuse the Panna Cotta: Bloom the gelatin in cold water. In a pot, bring the milk, cream, sugar, ½ cup of the strawberry-rhubarb puree, and crushed cardamom just to a boil. Remove from heat, stir in the orange zest, and let steep for 30 minutes.
Set the Molds: Whisk the bloomed gelatin into the warm cream mixture and strain through a fine-mesh sieve into 4 molds. Chill for at least 6 hours or overnight.
Poach the Fruit: Combine orange juice, water, sugar, and cardamom in a pan and bring to a bare simmer. Add the rhubarb batons and poach gently for 5 minutes. Add the strawberries during the last 2 minutes of poaching so they soften but hold their shape. Cool the fruit in the liquid.
Plating: Unmold the panna cotta in the center of the plate. Place a poached rhubarb baton and a few strawberry halves alongside. Add small dots of the remaining strawberry-rhubarb puree around the plate. Finish with a scoop of brown butter ice cream and fresh lemon balm.
