Tip ready meals onto a ceramic plate or into a glass dish before you microwave or oven-heat them. When you have the choice, pick meals sold in card or clear trays instead of black ones.
Heat makes plastic release more into food. A 2023 US study found that microwaving plastic food containers released far more tiny plastic particles than any other use it tested (doi.org/10.1021/acs.est.3c01942), and a UK study found hot oil pulled flame retardants out of black plastic utensils (doi.org/10.1016/j.scitotenv.2017.08.173). No study has yet measured what black ready-meal trays release in a microwave, so this is a sensible precaution, not a proven harm.
Black trays are also a waste problem. Recycling plants sort plastics with light sensors that cannot see the carbon black pigment, so most black plastic cannot be sorted for recycling and goes to landfill even when you put it in the recycling bin. The Australian Packaging Covenant Organisation (APCO) listed rigid plastic packaging with carbon black for phasing out (apco.org.au/news/20Y4a00000000PaEAI).
The catch: it takes a minute and an extra dish to wash, and the tray is still waste.