Lamb goes extraordinarily well with beans, especially things like flageolets. Soak overnight, then bring to a boil in a generous amount of mild stock with some peeled garlic cloves, half an onion, and a sprig of thyme. Reduce heat and simmer about 20 minutes or so until getting tender -- when you pull one out and blow on it, the skin peels back. Add a generous pinch of salt and some coarse pepper, and simmer another 20 minutes or so until just done. Allow to cool in the pot, and reheat gently if need be. For a fancier take on this, pull out a bunch of the beans and mash them with a fork in a bowl, then add some heavy cream and a little butter and keep mashing until the liquid is basically absorbed. The beans will not be smooth. Add the strained beans to this mix and serve hot.
Another good side for lamb is a spring-y fruit salsa. I like kiwi and melon, but that's because I usually end up getting my lamb in late summer, for complicated reasons. Pick what's really good: you want strong, bright flavors and a fair amount of acid. Add red or white onion, fresh mint chiffonade, seeded hot chile to taste, and salt and pepper. If your fruit isn't acidic, add a squeeze of grapefruit or lemon -- I find that lime doesn't work well here.
If the weather is very warm where you live and you have the option to grill a bit, try grilled fruit, e.g. apricot or peach. Peel, halve, and remove the seed. Roll in olive oil, then leave to drain. Just before grilling, sprinkle the cut side with coarse salt and pepper, then grill hot until the top is just warm, by which point the bottom will be well caramelized. Garnish with fresh herbs appropriate to the lamb. (I can't stand rosemary, but if you must use it, let it sit next to the fruit and encourage people to sniff it while they eat -- that way its flavor is present without overpowering everything.)