Over medium-low heat, sauté the leeks and the onion in melted butter a medium skillet until onions are transparent but not browned, which will take about 10 minutes; let cool.
In a medium bowl whisk together the beaten eggs, cream, crushed garlic, salt, white pepper, and lemon juice.
Add the cooled leek/onion mixture and mix well.
Spray six 4-ounce ramekins with cooking spray and pour/divide the pudding mixture equally into all the ramekins.
Bake the puddings at 350 degrees F for 15 to 20 minutes or until they have set and are lightly browned; let stand for 4 minutes before serving.
To serve, carefully unmold the puddings by inserting a small paring knife at the edge and circling around the ramekin wall, then invert onto warmed plates; garnish with sliced leek sprigs.