Mushrooms, in general, can release quite a bit of water when they are cooked.
If you happen to have one of those salad spinners you could use to spin the mushrooms once sliced to take all the excess water out.
Don't worry if you don't have one just let them dry for a few minutes.
Also one secret when cooking the mushrooms is that that they don’t release a lot of water is to cook them on a higher heat (uncovered) and not to add the salt until towards the end of the cooking.
Cooked in smaller batches it can also help. If you're having quite a bit of water in the pan when cooking the mushrooms, simply drain this into the lentils while cooking.
It will just give the lentils a little extra flavour which is quite nice.
For cooking the lentils I normally measure these with a measure which can be a cup or a little bowl and then use the same cup or bowl to measure the water.
Each amount of lentils will need the same amount of water plus another 1/2. For example for 1 bowl full with lentils will be the same bowl full with water plus another half.
Pretty straight forward, right?! If you don’t have or like the rocket salad you could easily use some kale, pak choi that you could add to the mushrooms in the last 3-4 min of cooking or could cook separately and just added to the dish.
We had this with some really ripen, juicy and incredibly flavoured 'ugly' tomatoes from the garden that expressed their beauty through their sublime taste.
If you don’t have any balsamic or don't like it, simply use generously freshly squeezed lemon juice.