Prepare the stuffing:
In a large sauté pan over medium-high heat, melt the butter. Sauté the fresh mushrooms and the porcini until nicely browned, about 7 minutes. Season with salt and pepper. Add the squash, shallots, garlic, and sage. Sauté over medium heat until the squash is al dente, about 5 minutes. Let cool. Transfer to a pastry bag without a tip or to a heavy-duty zip-top bag with a corner snipped off. Refrigerate until ready to use.
Prepare the veal:
Heat the oven to 425°F. Insert a long, thin-bladed knife straight through the center of the roast as far as you can, with the cutting edge to the right. Make a 3/4-inch cut to the right, turn the blade 180 degrees, and make a 3/4-inch cut to the left. Repeat, this time going up and down to create an incision that looks like a + sign. If your knife didn't go all the way through, repeat on the other side. Push the end of a wooden spoon through the completed channel to even out the enlarge the space. If the spoon is too short to go the length of the roast, repeat on the other side. With a pastry bag or zip-top bag, pipe half of the filling into one side of the roast. Repeat on the other side. The roast's diameter will almost double. Using a large needle and unwaxed dental floss, loosely sew across the ends of the roast, creating a mesh to hold in the stuffing. Leave the floss ends long for easy removal after cooking. Tie the roast with kitchen twine, spacing the ties about 1 inch apart.
Rub the veal with the olive oil, season it with salt and pepper, and transfer it to a small flameproof roasting pan or ovenproof skillet (you won’t need a rack). Roast until an instant-read thermometer registers 140°F, 20 to 30 minutes. When testing for doneness, be sure to insert the thermometer as far into the loin and as close to the stuffing as possible. Remove the floss and twine, tent the roast with foil, and let it rest for at least 5 minutes.
Make the sauce and serve:
Spoon off any visible fat from the roasting pan, leaving all the juices in the pan. Add the broth and the reserved porcini soaking liquid; bring to a boil, scraping up any browned bits with a wooden spoon. Continue cooking until the sauce is syrupy and reduced to less than half, about 10 minutes. Whisk in the butter and cream. Season with pepper. Carve the roast into even slices. Set a slice in the center of the plate, drizzle with sauce, and serve.
A really fine Pinot Noir will tie in all the earthy flavors. Look for these cool-climate Oregonians: Eyrie, Fiddlehead, or Domaine Drouhin.
nutrition information (per serving):
Photo: Grey Crawford