Cook the bacon. Preheat an oven to 425°F/220°C. Place 4 strips of thick-cut bacon (or more, if you want leftovers!) on a parchment lined baking sheet. Bake for 20-25 minutes, flipping the bacon once halfway through cook time, or until the bacon is crispy on all sides. Transfer to a paper towel lined plate or wire rack to drain excess grease.
Make the tarragon aioli. If you haven’t already, make the tarragon aioli. Season the tomatoes. Place the tomato slices in a single layer on a paper towel. Season lightly with flaky sea salt, then allow to drain while you continue with the recipe.
Make the blackening spice blend. Make the blackening seasoning blend. In a small bowl, combine 1 tsp dried oregano, 1 tsp paprika, 1 tsp Kosher salt, ½ tsp dried thyme, ½ tsp garlic powder, ½ tsp onion powder, ½ tsp crushed red pepper flakes, and ¼ tsp black pepper. Stir well to combine.
Prep the salmon. Pat the salmon dry with a paper towel, then drizzle with 1 Tbsp neutral oil. Sprinkle the blackening spice evenly onto both sides of the fish. Set aside.
Pan-sear the salmon. Heat a large skillet over medium-high heat, then add remaining 3 Tbsp neutral oil. When very hot, add seasoned salmon fillets, then cook for 4 minutes undisturbed. Use a fish spatula to flip the fish. If it does not release easily, it’s not ready to flip. Cook for an additional 3-4 minutes, or until an internal temperature of 120°F/49°C is reached.
Assemble the salmon sandwich. If you like, toast the bread. If not, begin assembling. Place heaping spoonfuls of the tarragon aioli on each side of the bread. Place 1-2 romaine lettuce leaves on the sauce, followed by a seasoned tomato slice, 1 piece of crispy bacon (break it in half to fit the bread), and a pan seared salmon fillet. Gently press the top bun on top. Serve immediately!