Combine the heavy cream and milk in a saucepan over medium heat. Add the fresh hay and bring the cream mixture just up to a simmer. Turn off the heat, cover, and let the hay steep for at least 30 minutes. Use a fine mesh strainer to strain the hay from the cream into a clean saucepan.
Meanwhile, in a small bowl combine the egg yolks, sugar, cornstarch, and salt. Whisk together well until the mixture begins to lighten in colour and texture. Set aside.
Reheat the hay infused cream over medium heat until hot. Just before the cream begins to boil remove it from the heat. Gradually temper the hot cream into the egg yolk mixture. Add a small splash of the hot cream and whisk it in completely before repeating. Do this until most of the cream has been added.
Return the mixture to the saucepan and continue to cook over medium heat, stirring constantly. Bring the custard just up to a boil and then turn the heat off. Add the butter and whisk together. Pour the custard into a bowl and cover with a layer of plastic wrap directly on the surface of the custard. Let cool completely. Store in the fridge.
Rhubarb and Apple Crisp
Preheat the oven to 375°F. Spray an 11" by 7" baking pan with non-stick spray and set aside. In a mixing bowl, combine the quick oats, brown sugar, 3/4 cup flour, and 1 tsp cinnamon. Add the melted butter and blend together to make a crumble. Set aside.
Peel and slice the apples and dice the rhubarb. In a mixing bowl, combine the apples and rhubarb and drizzle with the lemon juice. Add the sugar, the remaining 3 tbsp flour, and 1/2 tsp cinnamon. Stir together until the dry ingredients coat the fruit.
Transfer the fruit to the prepared baking dish. Top with the crumble and bake for 45 to 50 minutes until the fruit is bubbly around the edges and fully cooked.
The hay custard can be served either cold or warmed up. Top each serving of rhubarb apple crisp with a generous dollop of the hay custard and enjoy!