In medium saucepan, heat oil over medium heat. Add onion and cook, stirring often, until softened and translucent, about 5 minutes. Add squash and vegetable stock. Bring mixture to a simmer; cover and let cook until squash is easily pierced with a fork, about 8 to 10 minutes. With fork or potato masher, mash squash into a chunky paste. Stir in lentils, tomato paste, soy sauce, herbs, ground flax, and oats. Cook, stirring often, until mixture is well combined, warmed through, and thickened, about 4 minutes. Remove from heat.