Core tomatoes and cut a shallow X in bottom of each with a knife. Fill a bowl with ice and cold water. Blanch tomatoes in a large pot of boiling water for 10 seconds. Remove with a slotted spoon and immediately transfer to ice water. Peel tomatoes, then halve horizontally. Squeeze tomatoes over a bowl to remove seeds.
Pulse pepper, squash, vinegar, olive oil, and salt in food processor 2 or 3 times, until very coarse. Cut tomatoes into large chunks, add to food processor along with dill, and pulse several times until combined but still slightly chunky. Serve immediately or chill until ready to serve.