Capers (Capparis spinosa) contain one of the highest natural concentrations of quercetin. Capers flower buds of the plant, contain 173 milligrams of quercetin in every 3.5 ounces, or 14.8 milligrams in a typical 1-tablespoon serving.
Also all types of onions are high in quercetin. Raw red onions contain 33.4 milligrams of the compound in every 3.5 ounces, while raw white onions supply 21.4 milligrams, scallions have 18.3 milligrams and fresh sweet onions contain 14.8 milligrams.