To freeze: In large bowl, mix beef, feta cheese, bread crumbs, 1 of the eggs, the garlic, mint, salt and pepper; mix well. Shape mixture into 1-inch meatballs (about 36). Place on parchment-lined cookie sheet in single layer, and freeze 30 to 45 minutes or until firm. Transfer frozen meatballs along with carrots and orzo to 1-quart resealable freezer plastic bag. Add spinach to another 1-quart resealable freezer plastic bag. Freeze both bags up to 3 months. When ready to serve: In 4-quart saucepan, heat broth to boiling over high heat. Reduce heat to medium; stir in frozen meatball mixture. Simmer 10 to 12 minutes, stirring occasionally, until meatballs are cooked through and orzo is tender. Stir in frozen spinach; return to simmering. In bowl, beat remaining 2 eggs and lemon juice with whisk until frothy; slowly add 1 cup of the simmering stock, and beat combine. Transfer back to saucepan. Cook and stir about 2 minutes or until thickened. Garnish with cheese.