Unroll the spinalis so the meat is lying flat. Place it in the freezer for about 25 minutes to make it easier to slice.

While the beef chills, slice the onions into half-moons and preheat the griddle to about 350°F.

Remove the beef from the freezer and use a sharp knife to shave it as thinly as possible.

Place the shaved beef in a mixing bowl. Add the avocado oil and toss to coat. Season with salt, black pepper, and garlic powder. Let sit for about 15 minutes.

Add the butter and a little avocado oil to the griddle. Add the onions and sauté for 8–10 minutes. Season with a little salt, then move them to a cooler section of the griddle to keep warm.

Slice and toast the hoagie rolls.

Increase the cooking area of the griddle to 400–425°F. Add the shaved beef and cook for about 90 seconds.

Toss and chop the beef with a spatula, then continue cooking until the meat is cooked through.

Divide the beef into long rows. Top each portion with sautéed onions, provolone cheese, and pickled jalapeños.

Close the griddle lid and allow the cheese to melt. A few tablespoons of water can be added to the griddle to create steam and help melt the cheese.

Place the toasted hoagie rolls directly over the portions of beef. Slide a spatula underneath each portion and flip the meat and cheese into the rolls.

Add mayonnaise if desired and serve immediately.