For the Filling:

3 tablespoons unsalted butter

¼ cup finely chopped shallots

10 ounces button mushrooms, sliced ½ inch thick

2 teaspoons minced garlic (about 2 cloves)

½ teaspoon crushed red pepper flakes

10 ounces frozen chopped spinach, thawed and squeezed dry

Kosher salt, to taste

Freshly ground black pepper, to taste

For the Beef:

1 whole beef tenderloin (about 5 pounds), trimmed and butterflied

Kosher salt and freshly ground black pepper, to season

Butcher’s twine, for tying

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In a large skillet, melt the butter over medium-high heat until it starts to foam.
Add the shallots and sauté for about 5 minutes, until softened and fragrant.
Add the mushrooms and cook, stirring occasionally, until they release their liquid and begin to brown — about 7 minutes.
Stir in the garlic and red pepper flakes; cook for 30 seconds, just until aromatic.
Add the spinach, season with salt and pepper, and cook for another 2 minutes until everything is well combined and warmed through.
Remove from heat and let the mixture cool slightly before stuffing.

Lay the butterflied tenderloin flat on a cutting board.
Season both sides generously with salt and pepper.
Spread the spinach and mushroom filling evenly over the surface, leaving a ½-inch border along the edges.
Roll the tenderloin tightly into a log and tie it securely with butcher’s twine at 1-inch intervals.
Season the outside once more with salt and pepper.
Let it rest at room temperature for 45 minutes while you prepare your grill or oven.

Grill Method (recommended):

Preheat your grill for indirect medium heat (about 375°F).
Place the tenderloin on the cooler side of the grill, cover, and cook until the internal temperature reaches 120°F (rare). This should take about 25–30 minutes, depending on the thickness.
Move the beef to the hot side of the grill and sear for 2–3 minutes per side until it’s well browned and reaches 130°F for medium-rare.

Oven Method (alternative):

Preheat oven to 425°F (220°C).
Place the rolled tenderloin on a wire rack set over a baking sheet.
Roast for 35–45 minutes, or until the internal temperature reaches 130°F for medium-rare.
Optionally, sear in a hot skillet with a little oil for a caramelized crust.

Transfer the roast to a cutting board and let rest for 15 minutes before slicing.
Remove the twine and slice into 1-inch thick rounds.
Serve warm with your favorite sides — roasted vegetables, mashed potatoes, or a rich red wine sauce.

For the best results, let your beef tenderloin come to room temperature before cooking — it ensures even doneness.
Don’t skip resting the meat! This is key to keeping the juices inside.
You can prepare the stuffed and tied tenderloin a day in advance — just wrap it tightly in plastic and refrigerate until ready to cook.