In a medium bowl, whisk to combine the flour and brown sugar. Use a pastry cutter, two forks or your fingers to cut the cold butter into the flour and sugar mixture until it forms small pebble sized crumbs. Some can be large and some smaller. Place in the fridge to chill until needed. 

Preheat oven to 350 F. Lightly butter or spray  then line an 8×8″ pan with parchment paper. Leave some excess parchment paper hanging over two sides to make it easier to remove the bars once baked and cooled.

Add the graham crackers, walnuts, sugar and salt to the bowl of a food processor fitted with the metal blade. Pulse until mixture resembles the size of bread crumbs. Pour in the melted butter and pulse until combined.

Use your fingers or the bottom of a one cup measuring cup to press the mixture into the prepared pan and bake for 8-9 minutes til just lightly toasted. Remove from oven and allow to cool completely.

In the bowl of a stand mixer or in a large mixing bowl using a hand mixer, beat the cream cheese and sugar together on low until fully combined and smooth. Avoid beating the mixture at a high speed since that would add too much air into it. Mix in the eggs one at a time followed by the lemon zest and juice, honey and sugar until smooth. Gently stir in the chopped rhubarb and blueberries. Spoon the mixture over the cooled crust. Sprinkle the crumble evenly overtop.

Bake at 325 F for 35-40 minutes until the topping is lightly browned and the middle is still just a little bit jiggly when you lightly shake it. Remove from heat and let cool at room temperature for 45 minutes then place on a towel in the fridge uncovered to cool for at least 4 hours.