Bring a heavy pan over high heat to near smoking point, sear the red onion and leek cut side down and drizzle with the extra virgin olive oil. Reduce the heat to medium-high and cook, turning the vegetables until evenly charred on all sides, about 10 to 12 minutes. Remove from heat and allow to cool slightly and shop into half inch pieces.
Return the same pan to medium heat and add the bacon, cooking until lightly browned, about 10 minutes. Add the celery and cook until tender, about 5 minutes. Add the garlic, and cook an additional 2 minutes. Lower the heat and and add the chopped charred leek and red onion; cook a little longer for the flavors to meld, another 5 minutes, then turn off heat.
Start a large pot of water over high heat. When it comes to a boil, add enough salt to make the water taste like the sea. When the water is boiling, add the Tortelloni and cook according to the package directions. A minute or two before the pasta is done cooking, return the pan to medium heat, and add the cream. Then, add the Tortelloni to the pan, and stir gently. Add lemon juice and lemon zest and season to taste with salt and pepper.