Assiette vs. plat
While both plat and assiette can be related to dining in French, they are not interchangeable. Assiette refers to the physical plate you eat from, while plat most commonly refers to the food itself, the dish or course.
Assiette
A1An assiette is the physical, individual plate that a person eats their food from. It is the item of crockery itself.
J'ai besoin d'une assiette propre pour le dessert.
(I need a clean plate for dessert.)
Attention, l'assiette sort du micro-ondes, elle est très chaude.
(Be careful, the plate is coming out of the microwave, it is very hot.)
Il a léché son assiette tellement c'était bon.
(He licked his plate clean it was so good.)
Pour la soupe, on utilise des assiettes creuses.
(For soup, we use deep plates (soup bowls).)
Mon fils a accidentellement cassé une assiette en débarrassant la table.
(My son accidentally broke a plate while clearing the table.)
Plat
A1A plat primarily refers to a prepared dish or meal. It can also mean a course in a meal (like the main course), a large serving dish (a platter), or it can be used as an adjective for flat.
Quel est le plat du jour ?
(What is the dish of the day?)
La paella est mon plat espagnol préféré.
(Paella is my favorite Spanish dish.)
Après l'entrée, le serveur nous a apporté le plat principal.
(After the appetizer, the waiter brought us the main course.)
Peux-tu mettre les pommes de terre dans le grand plat bleu pour les servir ?
(Can you put the potatoes in the big blue dish to serve them?)
La campagne dans cette région est très plate.
(The countryside in this region is very flat.)
Ma mère prépare un excellent plat de lasagnes.
(My mother makes an excellent lasagna dish.)
Summary
In essence, you serve a plat (the food/dish) onto an assiette (the physical plate). Think of the assiette as the canvas and the plat as the painting. An assiette is what you hold, while a plat is what you eat.