Prepare the orange mustard: simmer sliced oranges with the sugar over low heat, stirring often, until thick and pulpy. Allow to rest and repeat a brief simmer if needed for full dissolution. Stir in vinegar and mustard extract, reduce lightly, then strain and chill overnight.
Blanch the red onion briefly, drain and marinate in a little vinegar and salt. Halve the tomatoes and remove seeds. Fill with orange mustard, garnish with onion and chopped olives, then finish with a drizzle of basil oil.
Montenidoli Vernaccia di San Gimignano Tradizionale 2021 — citrus, almond, and wild-herb notes echo the orange mustard and brighten the savory olives.