Baking – Preheating Onida convection microwave oven

Convection-baking in a "convection microwave" is a bit like conventional baking in a toaster oven; it's more or less the same principle but you really can't expect the same results as a conventional oven (or full-size convection oven).

That being said, if you're attempting the recipes in a convection microwave or even a real convection oven without any modifications, you may be using too high a temperature, or cooking for too long. The guideline for conversion (and this is just a guideline, not a rule or guarantee) is to subtract 25-30° F / 10-15° C from the temperature, and 25-30% from the cooking time.

So, for example, if a recipe calls for baking at 350° F for 60 minutes, your first attempt in a convection oven should be 325° F for 45 minutes.

Again, please keep in mind that a convection microwave is still fundamentally a microwave oven. No matter what their marketing claims say, it is not a substitute for a conventional/convection oven and you are not going to see equivalent performance.

Assuming you do not have access to the instruction manual, and/or the manual does not specify whether it is safe to use the glass in convection/grill mode, I would assume not.

Are there any labels and/or writing embossed on the glass?

As I described in this similar question, "regular" soda-lime glass is not heat safe. Given that the glass was obviously designed for a microwave, it is probably tempered glass, so it can withstand some amount of heat. Therefore, I would treat it like a Pyrex baking dish and not heat it to over 500°F. Your microwave's convection mode likely does not get that hot, so you are probably fine with that, but no guarantees. I would not use the tray in grill mode because even Pyrex is not broiler-safe.