How to add savor to a squash and grain mix?

It looks to me like you need umami. One easy, healthy thing you can add is powdered dried shiitake or porcini mushrooms. I just throw the dried mushrooms into a spice grinder, it's a powerful punch.

EDIT: (SAJ14SAJ refers to the same concept, glutamates, in his answer)

While I agree with some of the other answers that glutamates and nucleotides will help enhance the flavor of your dish, I don't think that's where you need to start.

As is, your "health mash" barely has any flavor to enhance. I'd start instead by adding some aromatics. You'd be amazed how much more flavor you'll get if you just add some sauteed or caramelized onions, roasted garlic, shallots, leeks, minced ginger, etc. Adding aromatics will give you a lot more depth of flavor which can then be enhanced with salt and umami-enhancing compounds, and further brightened with acidity.

There are three main things that are going to add or enhance the flavor of food. Salt, sugar, and glutimates.

This is why the restaurant trio of salt, butter, and bacon is so effective at making things taste good. Cheese is another ingredient that brings most of these factors to the table, especially hard aged cheeses like Parmesan. Tomatoes also help with glutimates, the so called umami flavor.

Secondarily, a touch of acidity may also help bring out flavors, as from vinegar, lemon or lime juice, or even tomatoes (there is a reason they are so popular). Some people may also enjoy spiciness as from hot chili peppers, mustard, horse radish, or black or szchuan peppercorns to help enhance flavor.

These are the types of ingredients you need, although you may choose not to use them for reasons other than flavor.

Depending on how you feel about animal fats (not exactly leaning towards vegetarian, but you did say "mostly vegetarian"), but you could pick up some salt pork at the grocery store. Dice it, and put it in a pan over a low heat to render out the fat. Use some of that fat when you're cooking the lentils, quinoa, and squash. Or just add it to the finished mash. (Also, try adding some lime juice or white wine when you're cooking the quinoa, too)

Whether you use the meat and crispy fat bits from the salt pork is up to you. The fat will be pretty salty, and will add quite a bit of that umami mouth feel to your dish.

If you have any objections to pork, or pork fat, you could pick up "Duck Bacon" at your local higher end grocery store, and accomplish similar feats. Render the fat, reserve the crispies, and use whatever you need to get the taste where you want it to be.

I am 100% convinced the solution for you is: savoury miso paste. And given your philosophy of eating, I would go the extra mile and find proper fermented miso (so you get the lacto-bacillus health effects). It is salty by nature, so adjust the other ingredients. Being a paste, it should be easy to include, but you can easily loosen it with a bit of warm water to mix it in more easily. You can find it in health food stores, and japanese groceries. It's also an excellent baste or marinade for fish/meat, and loosened a bit then add sesame oil for a great dressing for veg.