Step 1: Bring a large pot of water to a boil. Use a knife to score a small 'X' on the bottom of each tomato. Submerge the tomatoes in boiling water for 30-60 seconds, then immediately transfer them to an ice bath.
Step 2: Gently peel the skins off the tomatoes and discard. Roughly chop the peeled tomatoes, making sure to reserve all the juice.
Step 3: In a large saucepan or Dutch oven, melt the butter over medium heat. Add the onion, celery, and bell pepper. Sauté for 5-7 minutes until the vegetables are tender and the onion is translucent.
Step 4: Add the chopped tomatoes and their juices into the pot. Stir in the sugar, salt, pepper, and dried basil if using.
Step 5: Bring the mixture to a gentle boil, then reduce the heat to low. Simmer uncovered for 30 to 45 minutes, stirring occasionally, until the tomatoes have broken down and the liquid has thickened to your desired consistency.
Step 6: Taste and adjust seasonings if necessary. Serve warm as a side dish or store for later use.