Melt butter in a large saucepan over medium heat. Add onions to pan; cook 3 minutes, stirring occasionally. Add peas, broth, and 2 cups water; bring to a boil. Reduce heat, and simmer 10 minutes or until peas are very tender, stirring occasionally. Remove from heat; let stand 15 minutes. Stir in juice, salt, and 1/4 teaspoon pepper.
Place half of pea mixture in blender; process until smooth. Pour pureed soup mixture into a large bowl. Repeat procedure with remaining pea mixture. Pour half of pureed soup mixture through a sieve over a large bowl, reserving liquid; discard solids. Return liquid to pureed soup mixture. Ladle about 3/4 cup soup mixture into each of 6 bowls; drizzle each with 1/2 teaspoon oil. Sprinkle each serving with 1 teaspoon mint. Garnish with cracked pepper, if desired.
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