Preheat oven to 400°F (200°C). Line a baking sheet with parchment paper.
Place salmon on the sheet, rub with 1 tbsp olive oil, and season with salt and pepper.
Bake for 12-15 minutes until salmon flakes easily with a fork. Let cool for 5 minutes, then flake into bite-sized chunks.
In a large bowl, combine cucumbers, red onion, dill, and feta.
In a small bowl, whisk together extra-virgin olive oil, lemon juice, Dijon mustard, and minced garlic until emulsified. Season with salt and pepper to taste.
Add flaked salmon and diced avocado to the bowl with cucumbers. Pour dressing over and gently toss to combine.
Serve immediately or refrigerate for up to 4 days.