Some vegan recipes use yeast extract to boost umami flavour.
I have also heard of shiitake mushroom powder.
Another trick is to use anchovies, which is the trick used for Worcestershire sauce... or to use bacon, which is used in some hamburger recipes but then you would get smokiness as well.
Tomatoes and most mushrooms produce umami flavours when they have been cooked a certain amount.
Maybe dice some shiitake and cook that before the meat goes in the pan... I say "maybe" because I am not that familiar with that mushroom.
I would at least cook the tomato longer and replace some of the lemon juice with red wine.
BTW. Umami = glutamate. MSG = sodium and glutamate ions together as a salt.
Using natural sources of umami instead of adding pure MSG is chemically the same, except for the sodium.
So if you are one of the people who can get a headache from MSG, it does not matter where the glutamate comes from.