Bring well salted water to a boil in a large pot, then add troccoli and the broccoli rabe. Cook for about 5 minutes, or until the troccoli are al dente.
Meanwhile, in a large sauté pan or skillet, sauté the garlic cloves (and the peperoncino, if using) in abundant olive oil. Add the tomatoes if using and let them sauté for a minute or two until just slightly softened. Discard the garlic and peperoncino.
Add the reserved mussels and their juices. Let the juices reduce a bit if they're especially abundant. Turn off heat until the pasta is ready.
When the pasta is al dente, transfer it and broccoli rabe to the pan using a slotted spoon or wire skimmer, and give it all a good toss over a lively flame, adding a bit of pasta water if need be to keep things moist. Keep tossing until the pasta and its condimento are well mixed, and the pasta has absorbed most of the liquid.