Add flour, ⅓ cup of the flax, 2 Tbsp of the sugar, and sea salt to a food processor. Pulse a few times to combine. Add cold cubed butter and pulse again until it resembles coarse sand.
Turn on the food processor and stream in the ice water letting it go until a ball of dough forms and it pulls away from the edges (about 10 seconds). Form into a disc, wrap in plastic and chill for at least 2 hours (or overnight).
When ready to make, preheat the oven to 400 degrees and pull the dough out of the fridge. Letting it sit out while you assemble the filling will make it easier to roll.
In a medium bowl, combine strawberries, rhubarb, remaining 1 Tbsp of flax, remaining 2 Tbsp of sugar, vanilla and lemon zest and juice. Set aside.
Roll out the dough until about ⅛” thick (it doesn’t have to be a perfect circle!). Mound the fruit mixture in the middle leaving about 1.5-2” around the edge.
Gently fold the edge of the crust up and over the fruit, overlapping as necessary, but leaving fruit exposed in the middle (see photo). Optional: Brush edge with butter and sprinkle with coarse sugar.
Bake for 40-45 minutes until crust is browned and fruit filling is bubbly. Let cool on baking sheet for about 10 minutes before serving.