1. Toss rhubarb, strawberries, 3/4 cup granulated sugar, brown sugar, and orange juice in a large bowl. Let macerate for 30 minutes, then drain off the accumulated liquid.

2. In a separate bowl, whisk flour, 1 tablespoon sugar, and salt. Cut in cold butter and shortening until pea‑sized pieces remain. Gradually add ice water, 1 tablespoon at a time, until dough comes together. Divide in half, shape into discs, wrap, and chill for at least 30 minutes.

3. Roll one disc to a 12‑inch circle on a floured surface. Fit into a 9‑inch pie dish, leaving a 1‑inch overhang. Sprinkle 1 teaspoon sugar on bottom. Chill.

4. Roll second disc and cut into 1‑inch strips for lattice; chill strips.

5. Stir cornstarch into drained fruit mixture. Add strawberry jam and toss to coat. Spoon filling into bottom crust, leaving behind any liquid. Dot with small pieces of cold butter.

6. Weave lattice top: Lay 5 strips across the pie, fold back every other strip, lay a cross strip, unfold, repeat. Trim overhang to 1 inch, tuck edges under, and crimp.

7. Whisk egg yolk with 1 tablespoon water. Brush lightly over lattice; sprinkle with coarse sugar.

8. Place pie on a foil‑lined baking sheet. Bake at 425°F for 15 minutes, then reduce heat to 375°F and bake 45‑50 minutes more, until filling bubbles thickly in center and crust is deep golden.

9. Cool on a wire rack for at least 4 hours before slicing.