Clean the celery, washing off all dirt. Chop into desired size. I suggest cutting the ribs into approximately 1 inch pieces (I cut on the diagonal to try to make it look fancier). Include the ribs and as much of the leafy greens as you can--they're tasty too! (My cleaning approach for a particularly muddy head of celery was to chop first, discarding the dried out tops, dunk it all in a bowl of cold water and swish around to get the dirt to fall to the bottom, and then scoop out the celery pieces into a salad spinner to remove excess water.).
In a medium to large size saucepan on the stove top, saute the celery (you may need to work in two to three batches depending on your pan size and amount of celery) in the 1 tbsp extra virgin olive oil. (If you saute in batches, only use a total of 1 tbsp oil--e.g. if you saute in two batches use about 1/2 tbsp per batch.) When sauteing, try to develop a little of the browning/carmelization that will give the dish extra flavor by not stirring constantly. When the celery has cooked to your desired done-ness (e.g., I like mine lightly sauteed and still a little crispy), lightly salt and transfer to a 8 x 8 square glass baking dish. Repeat until all celery is sauteed and in the baking dish.
(Assuming you have not burned the celery in the pot) Using the same hot pot you just emptied of the last batch of sauteed celery, over medium heat add another 1 tbsp of olive oil and the diced onion. Saute for a few minutes to your desired done-ness. Add the 1 1/2 tbsp butter and allow to thoroughly melt, stirring in to the onions. Once melted, add the 1 1/2 tbsp flour and stir well. Keep stirring for a few minutes--not allowing to burn--as the roux comes together. (Just keep it moving in the pan, and take it off the heat if you think it is starting to burn.).
Slowly add small amount of the milk, stirring well after each addition to thoroughly incorporate. Add the nutmeg and taste for seasoning. You'll probably need to add more salt, maybe even a few tablespoons (depending on the type of salt you use). Keep stirring the mixture over medium heat until you get the seasonings right and the mixture comes just up to a boil and thickens. Turn off the heat.
Mix the melted butter with the bread crumbs.
Add the sauteed celery back in and stir to coat the celery with the creamy mixture. Transfer the creamy celery mixture back to the baking dish. Top with the butter/bread crumb mixture and bake (or broil if you pan can handle it) in the oven just to brown the bread crumbs.
Variations: You could add or substitute other vegetables in this dish, e.g., peas, carrots, potatoes, brussel sprouts, cubed squash, asparagus, leafy greens. You could also add herbs and spices to your liking. You could add shredded cheese to the roux once it comes together to give it a cheesier flavor. You can omit the bread crumb topping--I often do.