This rich, elegant soup is a perfect beginning for an important dinner. Although we use frozen peas and spinach with excellent results, the mint really must be fresh.
Melt the butter in a large heavy pot over low heat. Add the chopped onions, cover, and cook until tender and lightly colored, about 25 minutes.
Meanwhile, drain the spinach and squeeze out the excess liquid. Pour the stock into the pot, stir in the peas and spinach, and bring to a boil. Reduce the heat and simmer, partially covered, until the peas are really te