Place the baguette slices on a baking sheet. Brush them with olive oil, and lightly sprinkle them with salt. Bake for about 4 minutes, until golden around the edges, but not hard – they will firm up as they cool. Let them cool to room temperature.
Use a sharp knife to cut the thick bottom part from the stem, about the bottom two inches. Cut the remaining stalks, leaves and florets into slices about 1 inch across. You can chop it into smaller pieces if you like, which is nice if your broccoli rabe is large and mature, with bigger florets and leaves and thicker stems.
Heat the oil in a large pan with a lid. Add the onion, season with salt and pepper halved and sliced and saute for about 8 minutes until the onion is softened and browned. Then add the broccoli rabe and saute for 3 minutes.
Stir in 1/3 cup of water (or broth), cover the pan, and allow the broccoli rabe to cook for about 5 more minutes until it is done to your liking. Adjust seasonings as needed. Remove from the heat and set aside, pouring off any remaining liquid.
While the broccoli rabe is cooking, make the sauce. In a small bowl combine the mayonnaise and the chipotles in adobo, and season with salt and pepper.
Spread some Chipotle Mayo on each of the crostini. Pile some of the broccoli rabe, give it a light grind of pepper, then sprinkle over a tiny bit of lemon zest, if using.