Add the chopped onions, cover, and cook over low heat until tender and lightly colored, about 25 minutes.
Meanwhile, drain the spinach and squeeze out excess liquid.
Pour the stock into the pot, stir in the peas and spinach, and bring to a boil.
Reduce heat and simmer, partially covered, until peas are tender, about 20 minutes.
Pull the mint leaves from their stems; there should be 2 cups loosely packed leaves.
Rinse thoroughly and pat dry.
When peas are tender, add mint to the pot, cover, and simmer for another 5 minutes.
Pour the soup through a strainer, reserving liquid, and transfer the solids to the bowl of a food processor fitted with a steel blade, or use a food mill fitted with the medium disc (or just use the handy dandy blender).
Add 1 cup of the cooking stock and process until smooth.
Return pureed soup to the pot.
Add the heavy cream and additional cooking liquid, aboout 1 cup, until the soup is of the desired consistency.
Season to taste with salt and pepper, simmer briefly to heat through, and serve immediately.