Heat olive oil in a large saucepan over medium-low heat. Add onion and garlic; cook until translucent (5 minutes). Stir in cornstarch; whisk in chicken broth and evaporated milk; bring to a simmer. Stir in arugula and mixed chopped herbs until wilted; cover and set aside 5 minutes. Use an immersion blender to blend until smooth. Divide among 6 bowls; garnish each with 2 tsp plain Greek yogurt and 1 tsp sliced chives.