1. Heat a lightly oiled cast iron skillet over medium-high heat.
2. Pat the salmon dry and season with salt and pepper.
3. Add 1 tablespoon lemon garlic herb–flavored butter to the pan and let it melt.
4. Add the salmon filet skin-side down, reduce heat to medium, and cook for about 5 minutes until the salmon turns from pink to a lighter, opaque pink almost all the way through.
5. Flip the salmon, top with another tablespoon of butter, and cook for another 1–2 minutes until opaque throughout.
6. Remove salmon from the pan and discard the skin, if desired.
7. Toast the brioche bun under a broiler or in a toaster oven.
8. Spread the bottom half of the bun with the remaining butter.
9. Layer avocado, salmon, bacon, cucumber, tomatoes, pickled red onions, and arugula.
10. Top with the well-buttered bun and serve.

*Quick Pickled Red Onions

1. Place sliced onions and jalapeños (if using) in a 1-pint lidded jar.
2. Combine vinegar, water, salt, and sweetener in a small saucepan and bring to a boil.
3. Pour the vinegar mixture over the onions and press the onions down into the liquid.
4. Cool to room temperature, then cover and use or refrigerate.