Heat olive oil in a large skillet or Dutch oven over medium heat until shimmering.
Add the chopped onion and minced garlic to the pan, stirring until the onion becomes translucent and fragrant, about 5 minutes. The mixture should smell sweet and sound slightly sizzling.
Stir in the diced carrot and chopped celery, cooking for another 5-7 minutes until they soften and start to shimmer, releasing their sweet aroma.
Add the smoked paprika and cook for 30 seconds, stirring constantly to toast the spice and unlock its smoky scent.
Pour in the rinsed lentils and stir well, coating them with the aromatic vegetables and spices. Cook for a minute to lightly toast the lentils.
Add the crushed tomatoes and vegetable broth, stirring to combine. Bring the mixture to a gentle simmer.
Reduce the heat to low, cover loosely, and simmer gently for 25-30 minutes, stirring occasionally. The lentils should be tender but still hold their shape, and the sauce will thicken and deepen in color.
Uncover and cook for a few more minutes if the sauce needs to thicken further, stirring often.
Stir in the balsamic vinegar and season with salt and pepper to taste. Let the mixture sit off heat for 5 minutes to allow flavors to meld.
Give the ragu a final stir, then serve it hot over your favorite pasta, polenta, or crusty bread. Garnish with fresh herbs if desired for a bright touch.
