Preheat the pellet grill to 185°F (85°C).

Season the 1 1/2 lb steak generously with sea salt and cracked black pepper.

Insert the meat thermometer probe into the thickest part of the steak.

Smoke the seasoned steak at 185°F (85°C) for 60 minutes.

Thinly slice 3 large onions for the caramelized onion topping.

Melt 2 tablespoons butter in a skillet over medium-low heat.

Add the sliced onions, 2 tablespoons thyme leaves, 2 minced garlic cloves, and sea salt to the skillet.

Cook the onion mixture for 30 to 35 minutes, stirring regularly, until deeply golden and caramelized.

Mince 2 garlic cloves and 2 tablespoons chives for the garlic-chive mayo.

Combine 3/4 cup mayo, the minced garlic, juice from 1/2 lemon, and 2 tablespoons chives in a mixing bowl.

Season the garlic-chive mayo with sea salt and stir until smooth.

Thinly slice the remaining large onion and place it in a mixing bowl.

Pour 1 cup buttermilk over the sliced onion and soak for 10 minutes.

Heat enough neutral oil for frying in a skillet to 350°F (175°C).

Season 1 cup all-purpose flour with sea salt in a clean mixing bowl.

Dredge the buttermilk-soaked onion slices in the seasoned flour until well coated.

Fry the coated onion slices in batches until golden brown and crisp.

Transfer the fried onions to paper towel to drain excess oil.

Heat 2 tablespoons neutral oil in a clean skillet over medium-high heat.

Sear the smoked steak for 2 minutes on each side until browned.

Add 2 tablespoons butter to the skillet and baste the steak until medium-rare or your preferred doneness.

Rest the steak for 5 minutes before slicing thinly against the grain.

Slice and toast the baguette until crisp and lightly golden.

Spread garlic-chive mayo over each toasted baguette slice.

Layer sliced steak, caramelized onions, and fried onions over the crostinis.

Finish the crostinis with chives, a squeeze of lemon, and finishing salt before serving.