Peel and wash the asparagus and cut off the lower woody ends. Cut the carrots into thin slices. Wash the sugar snap peas. Bring 3/4 l salted water to the boil. Let the asparagus and carrots cook in it for about 10 minutes. Add the sugar snap peas just before the end of the cooking time. Drain the vegetables, collecting the vegetable stock. Put the tortellini in plenty of boiling salted water and cook in it for about 12 minutes. Drain.
Wash the chicken fillets, pat dry and cut into large cubes. Heat oil in a pan. Fry the meat all over and continue to fry for about 5 minutes. Season with salt and pepper and remove.
Add butter to the frying fat and let it melt in it. Sweat the flour in it until golden. Pour in the vegetable stock while stirring and bring everything to the boil. Add creme fraiche. Season with salt, pepper and lemon juice. Heat the vegetables, tortellini and chicken in the sauce. Place in a bowl and serve garnished with chervil.