Cooked-Down Onion for Crostini

I’d love to be able to tell you that I found the recipe for these crostini in a fifteenth-century Florentine manuscript written backward, but the truth is that the history of the dish begins in my own kitchen. I had got carried away slicing onion and made a lot more than I needed for something I was cooking. I cooked it all anyway, and to the surplus I added some diced tomato and chili pepper, processed it to a creamy consistency, mixed in some basil, and I had onion spread.