In a large heavy-duty plastic sealable bag combine lamb and 3/4 cup marinade and seal bag, pressing out excess air. Marinate lamb, chilled, for at least 4 hours and up to 8.
Blanch the bell peppers in a large saucepan of boiling water 30 seconds. Drain peppers in a colander and, while still hot, in a bowl stir together with 1/4 cup marinade. Marinate peppers, covered and chilled, for at least 4 hours and up to 8.
Prepare grill. Soak skewers in warm water 30 minutes.
Drain lamb, discarding marinade. In a sieve set over a bowl drain peppers and reserved bell pepper marinade for basting. Thread 4 pepper pieces and 4 lamb pieces, alternating pepper with lamb, onto each skewer. Kebabs may be assembled 1 hour ahead and chilled, covered.
Cover ends of skewers with aluminum foil so they don't burn.
Grill kebabs on a lightly oiled rack set 5 to 6 inches over glowing coals, basting with reserved marinade and turning them occasionally, about 12 minutes total for medium-rare meat. (Alternatively, broil kebabs under a preheated broiler 2 to 3 inches from heat about 4 minutes on each side.) Drizzle kebabs with remaining 1/4 cup marinade.
Mince together garlic, thyme sprigs, and rosemary with salt and mash to a coarse paste.
In a bowl whisk together garlic paste and remaining ingredients until emulsified. Marinade may be made 1 day ahead and chilled, covered.
Yield: makes about 2 1/2 cups
Recipe courtesy of Gourmet Magazine