Avocado-Jalapeno Soup

1/4 cup butter
1/4 cup all-purpose flour
2 cups milk
2 cups half-and-half
1 teaspoon salt
Grated rind of 1 lime
3 ripe avocados
1 tablespoon fresh lemon juice
1/2 teaspoon very finely chopped onion
1 teaspoon finely chopped jalape