Preheat your oven to 350°F and line a 6-inch square baking dish with parchment paper for easy removal.
Finely dice the fresh strawberries and rhubarb, then toss them in a small bowl with the lemon juice and honey until well coated.
In a medium mixing bowl, whisk together the rolled oats, vanilla protein powder, baking powder, and ground cinnamon.
Add the liquid egg whites and vanilla extract to the dry ingredients, stirring until a thick and slightly sticky dough forms.
Transfer two-thirds of the oat mixture into the prepared baking dish and press it down firmly into an even layer to create the base.
Spread the strawberry and rhubarb mixture evenly over the oat base, ensuring the fruit reaches the edges.
Crumble the remaining oat dough over the top of the fruit layer using your fingers to create small, textured clusters.
Bake for 22 to 25 minutes until the fruit filling is bubbling and the crumble topping has turned a light golden brown.
Remove from the oven and allow the bars to cool completely in the pan before slicing into squares to ensure they set properly.