In a large 13x9 pan mix together the garlic, balsamic, oil, fennel seed, salt and pepper. Place the flank steak in the pan rotating the steak from back to front with the marinade. Place in the fridge to marinate for at least 2 hours up to 24 hours.
When ready to grill the flank steak, preheat grill.
Serve grilled flank steak, rested for 10 minutes, cutting against the grain with a side of peperonata and pesto potatoes.
Peperonata
While the meat marinades, bring a large skillet over medium high heat with the olive oil.
Add the sliced onion and garlic and cook for 3 minutes.
Add the peppers and fennel and cook down until softened 6 to 8 minutes.
Add chopped tomato, oregano sprigs, sugar, salt and pepper stirring and cook for another 5 minutes. Remove from heat. Discard oregano stems.
Peperonata can be made the day before and reheated when ready to serve at room temperature or warm.
Just before serving add the splash of sherry vinegar.
Roasted Pesto Potatoes
While the grill is heating up preheat your oven to 450°F.
On a large sheet pan toss halved baby potatoes with olive oil and salt and cook cut side down for 30 minutes.
Once cooked, toss the potatoes in pesto and serve topped with shaved Parmesan cheese.