Proper preparation of veggies for an egg scramble
Those 4 vegetables (and fungi) all require different cooking times. Chop the onion and pepper into dices of roughly the same size, to ensure even cooking. Get them frying, with a little salt which will help to soften them, over a medium heat while you slice the mushrooms, then add those: it is more or less impossible to overcook a mushroom incidentally.

Once the onion has softened but still has some bite, chop the tomato and add that, then add your eggs more or less straight away - the tomato doesn't need much cooking.