Heat the oil in a large skillet over medium heat. Add onions and garlic, sauteeing until onions are translucent, about 2 minutes. Add mushrooms and cook for another 2 - 3 minutes until the mushrooms have started to soften.
Once mushrooms have started to brown, add chickpeas, tomatoes, artichokes, quinoa, paprika and saffron. Season with salt and pepper and stir everything together. Add broth and stir again to incorporate. Bring mixture to a boil, then cover and reduce to simmer for 20 - 25 minutes, until most of the liquid has been absorbed.
Remove the lid and stir in the green beans. Smooth the top of the paella with your spoon and arrange the sliced bell peppers on top. Cover again and cook for another 10 minutes.
When ready to serve, squeeze lemon juice on top and serve immediately!