Heat the butter and oil in a medium saucepan over medium heat until the butter melts. Add the garlic, onions, crushed red pepper flakes, 1/2 teaspoon salt and a few grinds of black pepper. Cook, stirring occasionally, until the onions are slightly softened but not golden, about 4 minutes. Add the chicken broth and 1 cup water and bring to a boil over high heat. Reduce the heat to keep the soup at a low simmer and simmer, partially covered, until the onions are tender, about 7 minutes. Add the peas and parsley and cook until the peas are just thawed and tender, about 3 minutes (the soup may not return to a boil). Transfer to a blender and blend until very smooth (be careful when blending hot liquids). Season with salt and pepper to taste.
Stir the mascarpone with the lemon zest and juice, 1 tablespoon water and a pinch of salt in a small bowl.
Serve the soup warm, with a dollop of lemon mascarpone and a little bit of chopped parsley.
The soup can be kept refrigerated in an airtight container for up to 3 days.
When blending hot liquid, first let it cool for five minutes or so, then transfer it to a blender, filling only halfway. Put the lid on, leaving one corner open. Cover the lid with a kitchen towel to catch splatters, and pulse until smooth.