Heat olive oil and butter in a large skillet over medium-high heat. Add onion and salt; saute until onion is translucent, about 5 minutes. Stir in garlic, herb mix, fennel, red pepper flakes. Add balsamic vinegar. Continue to heat until mixture appears slightly syrupy, about 5 minutes. Add tomatoes and tomato paste. Bring back to a simmer until heated through, about 10 minutes.
