Heat the olive oil in a large pot over medium-high heat. Once hot, stir in the onions and saute until softened, about 4 minutes. Toss in the red pepper, garlic, kale and some salt and pepper and continue to cook for 2 more minutes. Pour in the tomatoes and stir to combine, then add the chicken stock. Turn the heat to medium, cover with a lid and cook until the kale is silky and tender, about 15 minutes.