Take a Scotch-Brite Non-Scratch scrub sponge
Take a microwave safe bowl and fill it mid-way with water and add a slice of lemon.
Place the bowl in the micro wave.
Put the microwave on high for 5 minutes. (Allowing the lemon steam to cut through the grime)
Use the Non-Scratch scrub sponge to wipe clean the inside of the microwave.
Before & After