In a large pot, add the butter and warm over medium heat until melted. Add onion and garlic, cook until softened, about 5 minutes. Stir occasionally.

Add the asparagus and salt, stir well. Cook for an additional 5 minutes

Pour the chicken stock into the pan and increase the heat. Bring to a boil, and then reduce to a simmer. Simmer uncovered for 10-15 minutes.

Add the tarragon and stir.

In batches, add the soup into a food processor. Puree until the texture is smooth.

Pour the soup back into the saucepan. Add the cream and lemon zest. Heat through, or if the soup appears too watery, continue to simmer until the soup has reduced and thickened. Stir frequently to prevent burning.

Serve warm with torn mint leaves.