Heat a broiler to high with rack set 6-inches from element.

Broil peppers until charred on all sides, about 20 minutes total. Transfer peppers to a bowl, cover with plastic or a plate and let steam/cool.

Meanwhile, heat a cast-iron skillet over medium-high heat for 5 minutes.

Brush steak with a thin layer of avocado oil, then season with 1 teaspoon each salt and pepper.

Add steak to preheated skillet and cook until deeply browned on first side, about 3 minutes. Flip and cook another 3 minutes (it gets smoky, so a good vent if ideal) then transfer to top rack in oven and broil until an instant-read thermometer reaches 125ºF (for medium-rare); 5–6 more minutes.

Transfer steak to a plate, tent with foil, and let rest 10 minutes.

While steak rests, remove and discard skins and seeds from bell peppers, then cut into thick strips.

Place ciabatta halves on a baking sheet, cut side up. Brush 2 teaspoons oil on each cut side of bread; season lightly with salt and pepper. Broil until lightly toasted and golden brown, about 2 minutes, broiling halfway through to encourage even browning.

Spread pesto on cut sides of ciabatta and top with provolone. Broil until cheese is bubbly and starting to brown, 1–1 ½ minutes.

Slice steak, against the grain, into very thin strips.

Arrange half of peppers on bottom half of ciabatta, top with spinach, followed by steak slices, and finally remaining roasted peppers. Top with remaining half of ciabatta bread, press lightly to compress, then cut into 4 to 8 servings and enjoy.