4 lamb leg steaks (6 to 8 oz. each)
Salt and pepper
5 tbsp. olive oil
2 large red onions, halved through the core and thinly sliced
2 sprigs rosemary
¼ cup aged balsamic vinegar
¼ cup red wine
2 tbsp. (packed) light brown sugar
3 small (or 2 medium) firm zucchini
4 cloves garlic—3 chopped and 1 crushed
1 tsp. crushed red pepper
½ cup mixed finely chopped fresh mint and flat-leaf parsley
1 lemon, juiced (about 1/4 cup)
4 sprigs thyme
2 tbsp. butter
Bring the steaks to room temperature, pat dry and season with salt and pepper.
Preheat a cast-iron skillet or griddle pan over medium-high.
In another large skillet, heat 2 tbsp. oil, two turns of the pan, over medium to medium-high. Add the onions and rosemary, season and cook, stirring often, until the onions are very tender, about 20 minutes. Add the vinegar, wine and sugar and toss until the onions are coated and glazed, 5 to 7 minutes; discard the rosemary.
Using a vegetable peeler, peel long strips lengthwise down the zucchini to form ribbons (see our zucchini ribbons how-to). In a large skillet, heat 2 tbsp. oil, two turns of the pan, over medium-high. Add the zucchini ribbons and cook, tossing often, until crisp-tender, 3 to 5 minutes. Add the chopped garlic and crushed red pepper; season with salt. Toss until the garlic is fragrant, a minute more, then mix in the mint and parsley and 2 tbsp. lemon juice; remove from the heat.
Add the remaining 1 tbsp. oil, one turn of the pan, to the cast-iron skillet. Add the lamb and cook, turning occasionally, 8 to 9 minutes for medium-rare. Add the thyme, butter and crushed garlic. Using a large spoon, baste the steaks with the pan drippings, about 1 minute. Douse with the remaining 2 tbsp. lemon juice.
Serve the steaks on plates with the onions on one side and ribbons of zucchini along the other.