Rub the poblano pepper with olive oil. Roast over the gas flame of your stove, turning often, until the skin is black, 7 to 10 minutes. Place pepper into a paper bag and seal; set aside to cool, about 10 minutes.
Combine cucumber and onion in a bowl. Set aside.
Take cooled poblano from the bag and scrape off the burned skin. Remove stem and seeds; roughly chop the pepper.
Place tomatoes, poblano, jalapeno, garlic, lime juice, sugar, salt, pepper, and cumin into the bowl of a food processor and pulse until thick and not runny. Add cucumber and onion; pulse 3 to 4 times to incorporate. Refrigerate until chilled, about 30 minutes. Serve.