Pesto Pea Soup with Tortellini

0 Cook tortellini according to package directions.
1 While tortellini cooks, heat a large saucepan over medium-high heat. 
4 Add butter and when it melts, add onion, celery, and salt. Saute until onions are fragrant, 4 to 6 minutes. 
5 Add stock and bring to a simmer. Add peas and simmer just until peas are warmed through and tender, 2 minutes for frozen and 5 minutes for fresh. (Reserve a few cooked peas for topping the soup if you'd like.)
2 Remove soup from heat. Purée soup in 2 batches in a blender until smooth (use caution when blending because hot liquids expand). 
6 Pour puréed soup back into saucepan. Stir in pesto and lemon juice. Taste and season with some more salt and / or black pepper, if needed.
3 Divide cooked tortellini between soup bowls. Pour soup over top. Top soup with reserved peas, Maldon salt, and a drizzle of  olive oil.