Melt the butter in a large saute pan over medium-high heat. Add the onion and jalapeno, and cook until the onion begins to turn translucent, 3 to 4 minutes.
Add the mushrooms and garlic to the pan and cook while stirring for another few minutes until the mushrooms shrink down and begin to brown.
Add the lentils to the pan and stir until they start to brown, a minute or two. Add in the broth, along with the oregano and salt, and bring to a boil.
Turn down to a low simmer and cook uncovered until the lentils are tender, 30 to 40 minutes, adding 1/2 cup water or more broth if necessary.
Drain (and discard) liquid, and season lentil mixture with additional salt and pepper, to taste. Serve with recommended accompaniments.