Preheat oven to 400˚F (200°C). Cook pasta according to package directions; drain, reserving 1/4 cup (60 mL) cooking liquid. Return pasta to pot.
Meanwhile, heat oil in large skillet set over medium-high heat. Cook onion and garlic for 1 to 2 minutes or until slightly softened. Stir in spinach and salt. Cook for about 2 minutes or until spinach is wilted; set aside.
Add milk, cream cheese and mustard to pasta; cook over medium-low heat, stirring often, for about 2 minutes or until mixture is steaming and cream cheese starts to melt. Stir in sour cream and reserved cooking liquid until well coated. Stir in artichokes, reserved spinach mixture, capers, parsley, half of the chives, lemon juice and pepper. Remove from heat; stir in 2/5 of the Asiago cheese.
Transfer to greased baking dish. Top with remaining cheese; bake for about 25 minutes until golden and bubbling. Sprinkle with remaining chives.
For easy entertaining, pasta can be assembled and then baked just before serving.