In my Walkthrough and General Guide for Rune Factory Frontier, I wrote sections suggesting the easiest Cooked Dishes as well as those that did not require high skill levels or kitchen extensions. A dish such as 'Ice Cream', for example, can be made with only the smallest kitchen and very little skill.
For the player who has all kitchen utensils and expansions as well as a fair amount of skill, there are a multitude of Cooked Dishes that restore a good quantity BOTH of HP and RP, making it far easier to defeat Boss Monsters and other powerful enemies.
For example, there is the old Harvest Moon favourite, Gratin.
Gratin: 1272G (Lv. 2)
HP 150, RP 225, FX 150 sec. Max HP +50, Earth Resistance +35
Pizza always is another surprisingly powerful dish in Harvest Moon/Rune Factory:
HP 195, RP 400, FX 120 sec. Element Defence +20, Stun Defence
HP 190, RP 225, FX 120 sec. Max HP +85
If you look at the special effect for both Doria and Gratin, it is a boost of Maximum HP. During a battle, this can be invaluable, making the difference between victory and defeat.
These Recipes all require not only an Oven but the largest Kitchen upgrade as well as they use 5 to 6 ingredients.
They are only a few examples among many.