To small saucepan, add honey, serrano pepper, red pepper flakes, and sherry vinegar. Heat on medium-low heat for about 7 to 10 minutes, or until mixture starts to simmer. Remove from heat, add rosemary sprig, and allow it to infuse with peppers for a further 10 to 15 minutes. Strain through a fine mesh sieve into clean Mason jar, and reserve. Seal jar if making ahead of time.

Slice peaches in half and remove pits. Heat grill and brush each peach half with olive oil. Place flat side down and grill for 2 minutes, turn each half by 45 degrees and grill for another 2 minutes. Turn peaches over and grill for a further 1 to 2 minutes. Remove to 6 individual plates or large serving platter. Top each half with 1 Tbsp yogurt, and drizzle each with 1 tsp hot honey mixture. Finish with sprinkling of lemon thyme leaves. Reserve any remaining honey for other uses (see tip).