Chronic Urticaria: This condition is characterized by the recurrent appearance of hives without an obvious cause, and it can be triggered by various factors, including food. The fact that hives appear immediately after eating, regardless of the diet, and resolve slowly, fits the profile of chronic urticaria, which often has a mysterious onset and can be exacerbated by eating due to increased histamine release or other mechanisms.

Food-Dependent Exercise-Induced Anaphylaxis (FDEIA): Although this condition typically requires exercise in addition to food ingestion to trigger anaphylaxis, some cases might present with urticaria after eating, especially if the patient engages in any form of physical activity after meals.
Histamine Intolerance: This condition arises from an imbalance between histamine intake and the body's ability to metabolize it. Symptoms can include hives, especially after consuming histamine-rich foods or drinks.
Gastrointestinal Allergy or Intolerance: Certain conditions like eosinophilic gastroenteritis or food protein-induced enterocolitis syndrome (FPIES) can cause urticaria among other symptoms, though these are more commonly associated with specific food triggers.

Anaphylaxis: Although the question describes hives that resolve slowly, it's crucial not to miss anaphylaxis, a life-threatening condition that can initially present with urticaria and progress to more severe symptoms like respiratory distress, cardiovascular collapse, and even death if not promptly treated.
Hereditary Angioedema (HAE): This rare genetic disorder causes recurrent episodes of severe swelling, which can sometimes be mistaken for urticaria. HAE does not respond to standard treatments for allergic reactions and can be life-threatening if not properly managed.

Mastocytosis: A condition characterized by the accumulation of mast cells in one or more organs, which can lead to the release of histamine and other mediators, causing symptoms like urticaria, especially after eating.
Systemic Mastocytosis with Associated Hematologic Neoplasm: A rare variant of mastocytosis that can present with similar symptoms but is associated with a hematologic disorder.
Eosinophilic Disorders: Such as hypereosinophilic syndrome, which can cause a wide range of symptoms including skin manifestations like urticaria, though these are less commonly triggered directly by eating.