A little sweet and a little tangy, you can make this "butter" as spicy as you like by decreasing or increasing the number of hot peppers you use. Two small jalapeños with seeds made this just a tiny bit "warm"--not spicy at all for hot pepper fans.
Core the tomatoes and cut them into large pieces. Core the bell peppers and remove the seeds and membranes, and cut into large pieces. Remove the stems from the jalapeños (you can remove the seeds if you want practically no heat, but I leave them in.) Peel the onion and chop it coarsely.
Place the green tomatoes in a blender with 1/2 cup water. (If they won't all fit with plenty of room at the top, do them in two batches with 1/4 cup water.) Cover blender, and blend on low-medium speed until liquified. Pour into a large saucepan. Repeat with the peppers and onion with remaining 1/2 cup water and add to saucepan.
Bring to a boil and then turn down heat to keep it at a boil for 15 minutes. (Be careful--the longer this cooks, the more it "spits" at you!)
Reduce heat to a simmer and add the vinegar, sugar, mustard powder, and turmeric. Simmer until much of the excess liquid has evaporated, about 20-30 minutes.
Mix the flour and 1/4 cup water in a bowl until smooth. Stir in one ladleful of the tomato mixture and mix well. As you are stirring the pot, add the flour mixture gradually, making sure no lumps form. Increase heat a little if necessary to bring it back to a boil. Boil gently, stirring, for about 5 minutes. Add salt to taste.
At this point, you can also test for spiciness, sweetness, and tartness, and add cayenne pepper, sugar, or vinegar to taste. Be sure to boil it for a couple of minutes after any additions.
Place in jars and seal. Store in refrigerator. (This can also be canned using a water-bath method for 10 minutes and stored unrefrigerated.)
Makes about 3 pints.
I don't have the equipment to can, but if I did, I would follow the directions for water bath canning so that this can be stored longer on the shelf instead of in the refrigerator.