Position a rack in the lower third of the oven. Preheat the oven to 350°F. Butter and flour a 10 × 2-inch round cake pan or 9-inch square baking dish.
Prepare the streusel topping...
I have good news and bad news for you. Good first: this is a supremely tasty recipe. But to get to the end result you have to peel pearl onions, which on the scale of things you have to peel is a real pain in the tuchus. I thought shallots were bad. BUT, the recipe is worth it, and those little onions make all the difference for this recipe. Just peas and prosciutto? Too salty and meaty. The onions make it work, and they're lovely, mild, creamy, perfect. (As an aside, I've heard tell that some grocery stores stock frozen pearl onions, which are already skinned. If you're lucky enough to find these, buy them.)
I think it was Simon Hopkinson, author of Roast Chicken and Other Stories, who said to wait until peas are in season, then buy frozen ones. It may sound silly, but unless you have peas in your garden and can go out and pick them right before making this recipe (or know a farmer who can get you ultra-fresh peas), don't bother with fresh peas. You'll pay premium for them, and unless they're super-fresh (picked that morning) the natural sugars in the peas will have converted to starch, leaving you with flavorless, mealy peas. Frozen ones, however, are almost guaranteed to be sweet and tender. Flash freezing is miraculous.
And therein lies the magic of this recipe. Tiny white pearl onions, frozen peas, prosciutto. Serve this over risotto if you like. Or don't. It's just as impressive standing on its own with a slice of buttered rustic bread for a light spring meal.
Heat in a large skillet over medium heat:
3 tablespoons olive oil
Add and brown lightly:
24 pearl onions, peeled
3 tablespoons water
Cover and cook over low heat until the onions are tender, about 5 minutes. Stir in:
One 10-ounce package frozen baby peas
4 ounces prosciutto or ham, finely diced
Salt and black pepper to taste
Cover and cook until the peas are tender, about 3 to 5 minutes.