1. Melt butter in a large saute pan over medium heat and add onions. Cook until soft but not colored, about 4 to 5 minutes. Reduce heat to medium-low.
2. Add ribboned lettuce to pan with stock, salt and sugar and toss to combine.
3. Add peas, cover pan and cook, just until lettuce and peas are tender, about 5 minutes, but might take longer for more sturdy varieties of lettuce.
4. If using cream, stir in at end. Taste and add seasoning as desired. Serve immediately.