You can serve this quick saute tossed with pasta or as-is for a bold and veggie-packed side dish.
In a large pot, heat olive oil over medium-high. Add bell pepper, jalapeno, and red onion. Cook, stirring occasionally, until onion begins to soften, 3 minutes. Add broccoli rabe; season with salt and pepper and cook, stirring occasionally, until vegetables are crisp-tender, 4 minutes. Stir in vinegar and sugar and cook 1 minute. Serve warm or at room temperature.
To mellow its flavor, precook broccoli rabe pieces in boiling salted water until leaves are wilted, about 1 minute, then drain and cook as directed.