Step 1Arrange 4 or 5 leaves of romaine in the bottom of a large steamer basket, overlapping them so there are no gaps.
Step 2Place the peas in a bowl with the shallots, lemon zest and sage leaves. Toss to combine well and then arrange the peas in an even layer on top of the lettuce leaves.
Step 3Arrange another 4 or 5 leaves of romaine on top of the peas, overlapping them so there are no gaps and gently pressing down to make a tight fit.
Step 4Place the steamer basket over a pot of cold water and cover tightly. Heat over high heat and cook 3 minutes after the steam begins to appear. The peas will be crisp and juicy. Remove the top layer of lettuce and empty the peas into a serving bowl, discarding all the lettuce. Stir in the butter and season with the coarse salt. Serve immediately.