Preheat oven to 375 degrees F and place oven rack in middle. Spray an 8x8 baking dish with cooking spray and set aside.
In a large bowl, combine the rhubarb, sugar, honey, salt, and lavender paste. Mix well, then transfer to the prepared baking dish, spreading into an even layer.
In another bowl, mix the oats, flour, brown sugar, and salt. Add in the melted butter and stir until mixed well and crumbly clumps have formed.
Sprinkle the topping over the rhubarb mixture evenly, then bake 40-45 minutes, until topping is lightly browned and rhubarb mixture is bubbling.
Remove from oven and allow to cool for at least 15 minutes. Serve topped with a scoop of vanilla ice cream, if desired.
Store in an airtight container in the fridge for 3-5 days. Leftovers can be eaten cold or warmed in the microwave or oven.