Leave some lemon juice (from a fresh lemon) mixed with a cup of boiling water in the microwave and you will be able to remove encrusted dirt in no time!
The glass clean with a wire brush. The plate is tempered glass and often rough. By the enormous temperatures from for example bake pizza remains on the board and is almost impossible to get clean. Tried with vinegar, ammonia, hydrochloric acid, but almost no results. Almost ordered a new one and then tried it with a wire brush. This was a huge success and the plate was clean within 5 minutes.
I've been looking for a cleaner for the glass of the microwave. Scourer, ammonia and so tried, but it did not have the intended result. I ended up with Remover (A detergent i use to clean the floor) and it works best. Remover slightly diluted with water and then wrap in plastic and let stand for one day.