Over medium heat melt the butter in small skillet and continue to cook until its browned. Add the garlic clove to the browned butter and cook for about 1 minute, stirring frequently.
Next, add the leeks, celery and salt to the skillet and continue to cook, until softened, about 4 to 5 minutes, stirring occasionally. Once cooked, set aside.
Meanwhile, bring a pan of water to a boil. Add the peas to the boiling water and cook until tender, about 1 ½ to 2 minutes. Drain the water from the peas and place the peas into a food processor.
Add the crème fraiche and the cooked vegetables to the peas. Puree the veggies to your desired consistency.
Meanwhile lightly toast the slices of bread.
Place two pieces of prosciutto on each slice of bread, spread a large dollop of pea puree atop the meat and then top with fresh lemon zest. Serve warm or at room temperature.