Preheat oven to 425°F.
Cut leek halves into 2-inch lengths and then very thinly slice each piece lengthwise, yielding 2-inch-long strips. Toss the leek strips, peas, oil and salt in a medium bowl. Spread evenly on a baking sheet.
Roast for 15 minutes. Stir in tomatoes. Return to the oven and roast until the vegetables begin to brown, about 10 minutes more. Toss with oregano and serve.