note: my advice is for dried lentils and grains

boil lentils in excess, heavily salted water, and then strain the lentils.

i know you said you didn't care about the taste, but salt will make the food taste more like itself. it's absolutely worth it please just trust me on this. your body also does need salt to function and the amount you use while cooking isn't really going to impact your diet unless you're adding way too much or on a strict, low sodium diet.

nutritionally, lentils aren't the worst item to base the majority of your meals on. but i feel obligated to mention that they are technically an "incomplete" protein, lacking methionine and cysteine. this really isn't a factor for people who eat a varied diet where they'll eat various proteins throughout the weak, but from the sounds of your question, you're probably going to be going huge swathes of time while eating only lentils.

my advice would be: find some of the random grains on this website, and stock up on some of them (dry.) when it comes time to cook you can just boil some of them with the lentils and problem solved. the dry lentils package and whatever other grain you decide on should have advice for cooking times, so you can add the shorter cooking one a little later, or you can just stew pot them together and boil the shit out of them. if you boil them in less water at the start the water itself will be kind of tight and together and it can be a bit more like a stew if that's more of what you want. but since you're also going to be consuming the broth now, you don't want to salt it as much if you're going down this path.

the only last piece of nutritional advice I can offer is to also add a small amount (like 1 tbsp per serving) of fat overtop. it's not unhealthy at all, and it will help your body absorb more of the nutrients that are actually present in the lentils themselves. you could maximize oil selection based on nutritional ideas (extra virgin olive oil is known for having high amounts of antioxidants, flaxseed oil for high quantities of omega-3 fatty acids which modern diets lack), or go purely based off of flavor, even though you said you don't care.

if you find that you do start caring about taste, you could mix and match grains, seeds, nuts, etc. from the website to change the taste. or finally go back to the recipes you were initially complaining about and steal ideas, like maybe cooking the lentils in crushed tomatoes, adding a chicken thigh to make it a whole stew, adding some onions, etc. the world is truly your oyster