If you are really serious about this, you can calculate it.
First, obtain the macronutrient counts per 100 g of ingredient
you are adding. For example, lentils have 26 g of protein per
100 g. Second, weigh everything you are adding. Let's say you
want to cook 200 g lentils. Third, calculate the total amount of
nutrients you added. With my nice round numbers, it will be 52 g
proteins from the lentils. Fourth, weigh the prepared dish. (it
doesn't help to sum up the raw ingredients only, because water
will evaporate during cooking). Let's say you end up with 700 g
of cooked lentils. Fifth, calculate the new protein amount per
100 g. 52 g protein per 700 g lentils makes it around 7.4 g
protein per 100 g. Now you are ready. You only get a small
amount of error when some nutrients are destroyed by cooking,
for example a maillard reaction will involve both proteins and
carbohydrates as input and produce carbohydrates only. But it is
a very small error; the variability of nutrient levels between
batches of the same f