Slow-roast the salmon: Preheat the oven to 275°F. Arrange the fresh herbs and lemon slices on a parchment-lined baking sheet and place the salmon fillets on top. Drizzle with olive oil and season with salt and pepper. Bake for 25–35 minutes, depending on thickness, until the salmon is tender and flakes easily. For fully cooked fish, the center should reach 145°F.
Make the avocado cilantro dressing: Add the avocado, Greek yogurt, cilantro, jalapeño, lime juice, garlic, olive oil, salt, and pepper to a blender or food processor. Blend until smooth, adding water one tablespoon at a time until creamy and spreadable. Taste and adjust seasoning as needed. Divide the dressing into two equal portions.
Prepare the cabbage slaw: Add the shredded cabbage and scallions to a mixing bowl. Toss with the lime juice and half of the avocado cilantro dressing until everything is evenly coated. Refrigerate until ready to assemble.
Make the crispy shallots: Thinly slice the shallots into even rings. Add them to a small skillet with the neutral oil and heat over medium-low. Cook for about 10–15 minutes, stirring frequently, until lightly golden and crisp. Watch carefully toward the end, as they can burn quickly. Transfer to a paper towel-lined plate using a slotted spoon and immediately sprinkle with salt. They will continue to crisp as they cool.
Assemble the sandwiches: Lightly toast the ciabatta rolls or brioche buns. Spread the remaining avocado cilantro dressing generously onto the cut sides of the bread. Layer with the thinly sliced baby cucumbers, followed by the slow-roasted salmon, breaking it into large flakes if desired. Pile the dressed cabbage slaw over the salmon and finish with plenty of crispy shallots and fresh cilantro. Close the sandwiches and serve immediately.