Bring a large pot of lightly salted water to a boil; cook linguine at a boil until tender yet firm to the bite, about 11 minutes; drain and transfer linguine to a serving bowl.
Heat butter and olive oil in a saucepan over medium-low heat until bubbling; cook and stir onion and yellow bell pepper in the bubbling butter mixture until softened, 5 to 10 minutes. Stir cream cheese into onion mixture until melted. Add salmon and milk; simmer until cooked through, about 3 minutes.
Pour salmon sauce over linguine and toss to coat; garnish with dill and lemon slices.