Rice – How to achieve a good socarrat with paella cooked in the oven

Penelope Casas' The Foods and Wines of Spain explains that

Paella is a word that has come worldwide to mean a Spanish rice dish with a variety of seafood and usually some chicken. However, the word originally referred only to the pan in which the food was cooked--the paella or paellera, from the Latin word for pan, patella. Paellas actually come in endless varieties, depending on the chef and on regional specialties.

She emphasizes the technique more than traditional ingredients (which she notes that no two Spaniards will agree on), but includes recipes for a number of different rice dishes from many regions, all prepared in basically the same manner.

Interestingly, her Paella a la Valenciana (Tradicional) actually doesn't call for saffron (it does call for paprika). The meats are snails and rabbit, and she does call for an onion, but it's only used to steep for a while in the broth, and later discarded. She includes a single tomato which is cooked down with green peppers and garlic. Most of her other paella recipes include onion in this step, but it is notably absent from this recipe. She also calls for lima beans and "wide, flat string beans", and serve the dish with scallions on the side.

Oil or fat is absolutely not necessary to cook rice.

I suspect you may have been taught the pilaf method where the rice is first sauteed in oil or butter, and then liquid is added and the rice is fully cooked. The purpose of the pilaf method is to add depth of flavor. When making pilafs, additional herbs, spices, or aromatics (such as onions) are often included.

If you have been taught to add oil without the sauteeing, then the only purpose of it would be to add flavor. It won't really help prevent sticking, and isn't needed for the rice to cook properly.

There are many methods to cook rice in addition to the pilaf method, and many to do not require any fat. Simply boiling or steaming in water is completely traditional in many Asian cuisines.

In some cuisines, the crusty rice that sticks to the bottom of the pot or rice cooker is actually highly prized (see Wikipedia's nurunji article).

You can mitigate the crusting of the rice by making sure to use the lowest flame or heat setting that will keep the rice steaming. I actually prefer to put the whole pot in a moderate oven, rather than do it on the stove top.

You can also use the pasta method of simply boiling the rice in water, and then draining it when it is done to your liking, to completely avoid the crust. This will result in less sticky rice since the surface starches will be left in the boiling water. I understand this method is popular in some parts of India and Pakistan.