Make the Quick Pickled Onions: In a small saucepan, combine apple cider vinegar, water, sugar, and 1 tsp kosher salt. Bring to a simmer, stirring until dissolved. Pour the hot brine over thinly sliced red onions in a jar and let sit for at least 20 minutes.
Prepare the Avocado Mash: In a medium bowl, combine avocado flesh, fresh lemon juice, 1/2 tsp kosher salt, and black pepper. Gently mash with a fork until slightly chunky.
Toast the Bread: Drizzle olive oil in a skillet over medium heat. Toast sourdough slices for 2-3 minutes per side until golden brown and crisp.
Assemble the Toast: Spread a generous layer of the avocado mash over each slice of warm toast.
Add Salmon and Garnish: Drape 2 ounces of smoked salmon over the avocado. Top with pickled red onions, fresh dill, chives, a drizzle of olive oil, and a pinch of flaky sea salt. Serve immediately.
Notes
For the best results, assemble the toasts immediately before serving to prevent the bread from becoming soggy.