Heat oil in a large saucepan over medium-high heat. Add onions, garlic, 1 teaspoon salt, and a 1/4 teaspoon pepper. Cook, stirring, 3 to 5 minutes. Add stock, and bring to a boil.
Add peas, and return to a boil. Reduce heat, and simmer, stirring occasionally, until peas are tender and bright green, 2 to 3 minutes. Stir in spinach. Cover, and cook, stirring occasionally, until spinach has wilted, 2 to 3 minutes. Let cool slightly.
Working in batches, puree pea mixture in a blender, filling no more than halfway and adding up to 2 tablespoons water to each batch to achieve desired consistency. Stir in lemon juice, 1 teaspoon salt, and 1/4 teaspoon pepper. Ladle soup into bowls.
Fill a pastry or resealable plastic bag with cream. Snip off tip or corner of bag. Draw parallel horizontal lines across each serving of soup, spacing lines as desired.
Starting at bottom, drag the tip of a wooden skewer upward through the rows of cream. Starting at top and spacing as desired, drag skewer downward through rows of cream. Repeat, reversing direction every other line. Serve immediately.