In a large pot of boiling salted water, cook linguine 10 min or until al dente, adding asparagus, beans, and sugar snap peas during the last 3 minutes of cooking and edamame during the last minute. Drain very well.
Meanwhile in large saucepan, melt butter over medium heat. Add onion and garlic; cook, stirring 3 minutes. Stir in flour. Very gradually whisk in milk until smooth and stir often until mixture boils and has thickened (5 mins).
Add pasta mixture, stirring until hot. Stir in salmon and mint just until mixed. Squeeze juice from lemon into pasta and stir.