Counting carbs in cooking is something that has confused me until recently. To figure out carbs for a dish, you calculate how many carbs are in each of the ingredients you use. For example:
If the entire recipe has 1 cup of onion, 1 cup of tomato, 2 pounds lamb, 2 tablespoons mint leaves, 4 garlic cloves. You need to find the net carbs for each of these ingredients. Add them together. That will be the total net carbs for the entire recipe. Then you have to figure out how much the recipe made. For example, if the recipe makes 4 cups of stew and each serving is 1 cup, divide the total net carb amount by 4. That will give you the net carb per serving.
I cook similar to you: I use 2 tomatoes or 2 onions. But I will either weigh them or I will chop/slice and measure them.
Cooking green beans with potatoes and carrots won't give the green beans more carbs BUT the cooking liquid may have more carbs, especially if the potatoes begin to disintegrate into the cooking liquid.
Veggies are good for you, but some veggies negatively affect weight loss in people. For example, I can't eat any dish that combines eggplant and tomatoes because that combination gives me cravings, which makes me eat more, which screws up my weight. However, I can eat eggplant and tomatoes by themselves or in combination with other veggies, just fine.