Use singular for an ingredient if only one of an item is used. Use plural for an ingredient if more than one of an item is used. If ingredients such as cucumber, cabbage, onion, etc., are chopped or otherwise prepared, determine singular or plural based on how many units are needed to get the measured amount.
1 cup chopped onion (only one onion is needed to get 1 cup)
2 cups chopped onions (more than one onion is needed)
In method, maintain consistency with ingredients list.
1⁄4 cup chopped onion
2 tomatoes, chopped
Add onion, tomatoes, and carrots.
For seeds, use the plural form.
1⁄4 cup sesame seeds (not sesame seed)