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 3 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 3 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 5 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.