Heat oven to 375°. For easy removal, line bottom and two sides of 8-by-8-inch square baking pan with parchment paper. No need to bother (and no greasing needed) if you plan to serve them right in the pan.
Place oats, 3/4 cup flour, brown sugar and salt in bottom of baking pan and mix. Pour melted butter over, and stir until clumps form. If the clumps feel soft or look overly damp, add the remaining 2 tablespoons flour. Set aside 1/2 cup of the crumble mixture. Press the rest of the crumble mixture evenly in the bottom of the pan.
Spread half the diced strawberries and rhubarb over the crust. Sprinkle it evenly with cornstarch, then lemon juice, and 1/2 tablespoon of granulated sugar. Spread the remaining strawberries and rhubarb over this, and top with second 1/2 tablespoon sugar. Scatter reserved crumbs over fruit and bake bars for 30 to 40 minutes (firmer fruits will take longer), until strawberries and rhubarb are bubbly and crumby portion is golden and smells delicious.
Let cool in pan, preferably in the fridge. Cut into squares and sprinkle with powdered sugar before serving. Store leftovers in fridge.