I’ve eaten the flowers and the leaves both are fine, the leaves are better in the spring than later in the year. The berries although I’ve heard are edible raw are cooked and made into jellies mostly I’ve yet to try either.
Edible "berries", petals, and leaves
The fruit of hawthorn, called haws, are edible, but are commonly made into jellies, jams, and syrups, used to make wine, or to add flavour to brandy, rather than eaten fresh. Botanically they are pomes, but they look similar to berries. A haw is small and oblong, similar in size and shape to a small olive or grape, and red when ripe. Haws develop in groups of 2-3 along smaller branches. They are pulpy and delicate in taste. In this species (C. monogyna) they have only one seed, but in other species of hawthorn there may be up to 5 seeds.
Crataegus monogyna is one of the most common species used as the "hawthorn" of traditional herbalism, which is of considerable interest for treating cardiac insufficiency by evidence-based medicine. The plant parts used medicinally are usually sprigs with both leaves and flowers, or alternatively the fruit. Several species of Crataegus have both traditional and modern medicinal uses. It is a good source of antioxidant phytochemicals,especially extracts of hawthorn leaves with flowers.