In a medium pot, combine peas, butter, broth, and onions. If the peas are not submerged in liquid, add just enough water to cover them. Bring to a simmer over medium heat and cook for about 7 to 10 minutes, or until the onions are soft enough to puree and the peas are still bright green.
While the peas are cooking, grill the bread. Heat a grill pan over high heat for three minutes. Pour some olive oil onto a large plate. Sprinkle oil with salt then dip pieces of bread quickly on both sides into the oil. Grill bread until slightly charred and hot, about three minutes per side.
Using an immersion blender, standing blender*, or food processor, puree the soup until smooth and season with salt and freshly ground black pepper, to taste. Serve each bowl with a dollop of sour cream on top and grilled bread on the side.
*When blending hot liquids: Remove liquid from the heat and allow to cool for at least five minutes. Transfer liquid to a blender or food processor and fill it no more than halfway. If using a blender, release one corner of the lid. This prevents the vacuum effect that creates heat explosions. Place a towel over the top of the machine, pulse a few times then process on high speed until smooth.