Put the olive oil and butter into a large pot over medium-high heat. Add the vegetables and toss to coat them well with the fat; season with salt and pepper. Add 1 cup chicken stock and bring to a boil. Lower the heat to a simmer, cover the pot, and cook until the vegetables are tender, about 20 minutes. Add butter and toss until incorporated. To serve, pour vegetables in a large bowl and shower with mixed herbs and season with salt and pepper.