Preheat the oven to 350°F. Remove the green top and root section from the leek, and then slice the white section in half lengthwise. Rinse well under cold water, shake to drain out the excess liquid, and then cut ¼ inch thick slices. Set aside.
Heat a large skillet over medium heat. Add the olive oil then the leeks. Cook for 2 minutes, stirring occasionally, and then add the peppers. Stir in the garlic.
Continue to cook for an additional 7-8 minutes, allowing the veggies to soften and their flavors to meld together
As the veggies cook, prepare the salmon by arranging some of the dill on the bottom of a lightly oiled cast iron pan or Pyrex baking dish. Lay the salmon fillets on top, sprinkle with sea salt, and top with a lemon slice and cover with more dill. Place the fish in the oven and bake for about 8-10 minutes, just until the fish becomes slightly firm.
To finish the veggies, season with the salt and stir in the thyme. In a bowl, mix the arugula with the olive oil, lemon, and salt, toss well and serve.